There's not much you can do to stop a baby boy from peeing when you change their diaper.
But you can quickly place a towel or something over the baby boys penis to stop the pee from going in the air.
When changing the diaper place a new diaper underneath the baby before you remove the old diaper but before you open the old diaper have a towel or cloth ready to place over his penis.
Another thing you could do is slowly open the diaper and watch to see if he's peeing and if he is then put the diaper back on him and let him pee.
If you remove the diaper too quickly then he may pee in your face.
It's normal for babies to pee on you or just pee when you remove their diaper and there's not much you can do to stop it.
But you can put something over the penis to contain the pee.