Size 8 diapers are for ages 5 years old and above, usually up to around 10 to 12 years old.
Size 8 diapers are for kids that weigh 46 lbs and above and some kids may even be able to wear a size 6 or size 7 diaper at 5 years old to 7 years old.
Then after that they may need to move up to a size 8 diaper.
My 6 year old son also wets the bed and wears a size 8 parents choice diaper and they fit him just fine.
We tried the Goodnites, but they leaked on him as he's a heavy wetter and the tape on diapers like Parents Choice size 8 diapers work much better for him.
If your 6 year old son is a bit small for the size 8 Parents Choice diapers, you may need to use a size 7 diaper instead like Cuties size 7, Pampers size 7 or even Luvs size 7.
There's also other good brands of size 7 diapers that can fit your 6 year old son.
I used to fit into the old size 6 Pampers when I was a kid and wet the bed, but it can also all depend on the size of the 6 year old and their build.
A size 8 Parents Choice diaper and other size 8 diapers should even fit a 10 to 12 year old if they are of average sized build.
After that, you would likely need to switch to using Youth sized disposable diapers.