There's nothing wrong with him masturbating.
He's probably masturbating in the wet diapers because the wet diapers feel good against his penis.
I enjoy a good masturbation session in a wet disposable diaper. It feels good and there's nothing wrong with it.
Here's some more info that might help you.
What to do
Don't panic. Masturbation is a completely normal thing to do. It doesn't cause physical harm, pose a health risk, or mean your child is going to turn into a sex maniac. Masturbation in young children isn't sexual (as it is for adults) because young children don't know what sex is.
And although explicit sex play in older children is often a tip-off to sexual abuse or exposure to inappropriate sexual material, this is extremely unlikely to be the case with young children. They simply don't have the imaginative skills for this kind of behavior. (A young child who's been sexually abused is more likely to become withdrawn or suddenly have trouble sleeping.)
That said, young children masturbate because it feels good, and the good feelings can be as pleasurable for her as they are for adults. "A child may masturbate herself to orgasm," says Zweiback "complete with panting, red face, and a big sigh at the end. But it's absolutely not something to be worried about."
Of course, just like anything else, when it comes to masturbation too much of a good thing may indicate a problem. According to the American Academy of Pediatrics, if your child masturbates constantly or excessively, it may be a sign she's feeling anxious, emotionally overwhelmed, or isn't getting enough attention at home. If you think this may be the case, check in with her doctor for advice.
Ignore it. You may have already told your child that some of her parts are private, and that only she, her parents, or a doctor may touch them. Many parents attempt to explain privacy to children as a way to head off sexual abuse, and it seems logical to extend this concept to masturbation. But it may not sink in for your child.
"Privacy means nothing to an 'under 3,'" Meg Zweiback says. "It's not a meaningful concept." And, she adds, "A child this age by nature is looking to push buttons, so if you start drawing attention to it, you'll probably just get her to do it more." Your best bet is to look the other way or immerse yourself in a distracting activity.
Distract her. Even though you know it's normal and lots of children do it, you'll probably still be embarrassed if your child starts masturbating in front of company. If you can't ignore it or laugh it off, distraction is your best bet. Masturbation is a lot likenose-picking — she does it because she's bored, because her hands are free, and because she can.
If your child's hands stray toward her crotch at inopportune moments (in front of your in-laws, for example) keep a toy close by to give her instead. Invite her to do a puzzle, play with blocks, or toss a ball around – anything that keeps her hands out of her pants.
Look to yourself. Parents' reactions to masturbation may pose the greatest danger for kids. If your child is made to feel guilty for exploring her body, or made to feel that what she's doing is dirty or naughty, she may associate sexual or pleasurable feelings with guilt and shame.
"If a parent is really bothered by it," Zweiback says, "it says more about what the parent learned growing up than it does about the child. Lots of people grow up with conflicting feelings about sex, and finding a place where you can talk these feelings through with other adults will help you handle these issues now and in the future."
Why it happens
Young children masturbate for the same reason that older children (and adults) do: It feels good! Body exploration is part of growing up.
Your child is learning to run, jump, throw, pump a swing, draw, and (probably) use the toilet regularly. She /he may be just as curious about her genitals as she is about her fingers, toes, and belly button – and if she's recently switched from diapers to underpants, she may be able to get to them for the first time.
"When parents first see this kind of exploration, they wonder 'is this normal?'" says Meg Zweiback, a nurse practitioner and family consultant in Oakland, California. "The answer is yes. You don't need to be concerned."
What to do
Don't panic. Masturbation is a completely normal thing to do. It doesn't cause physical harm, pose a health risk, or mean your child is going to turn into a sex maniac. Masturbation in young children isn't sexual (as it is for adults) because young children don't know what sex is.
And although explicit sex play in older children is often a tip-off to sexual abuse or exposure to inappropriate sexual material, this is extremely unlikely to be the case with young children. They simply don't have the imaginative skills for this kind of behavior. (A young child who's been sexually abused is more likely to become withdrawn or suddenly have trouble sleeping.)
That said, young children masturbate because it feels good, and the good feelings can be as pleasurable for her as they are for adults. "A child may masturbate herself to orgasm," says Zweiback "complete with panting, red face, and a big sigh at the end. But it's absolutely not something to be worried about."
Of course, just like anything else, when it comes to masturbation too much of a good thing may indicate a problem. According to the American Academy of Pediatrics, if your child masturbates constantly or excessively, it may be a sign she's feeling anxious, emotionally overwhelmed, or isn't getting enough attention at home. If you think this may be the case, check in with her doctor for advice.
Ignore it. You may have already told your child that some of her parts are private, and that only she, her parents, or a doctor may touch them. Many parents attempt to explain privacy to children as a way to head off sexual abuse, and it seems logical to extend this concept to masturbation. But it may not sink in for your child.
"Privacy means nothing to an 'under 3,'" Meg Zweiback says. "It's not a meaningful concept." And, she adds, "A child this age by nature is looking to push buttons, so if you start drawing attention to it, you'll probably just get her to do it more." Your best bet is to look the other way or immerse yourself in a distracting activity.
Distract her/Him. Even though you know it's normal and lots of children do it, you'll probably still be embarrassed if your child starts masturbating in front of company. If you can't ignore it or laugh it off, distraction is your best bet. Masturbation is a lot like nose-picking — she does it because she's bored, because her hands are free, and because she can.
Source: http://www.babycenter.com/0_masturbation_11558.bc?PageSpeed=noscript