A 7 year old boy should be able to wear a size 6 diaper or size 7 diaper.
However if the 7 year old is a bit bigger than they may need a Youth sized disposable diaper which you can buy online.
Attends makes Youth Attends disposable tab style diapers which are better than the pull ups and are bigger than the size 6 or size 7 toddler diapers are.
If the 7 year old boy is skinny enough then size 6 Pampers Baby Dry diapers should fit the 7 year old just fine.
I have a 7 year old son that wears pampers baby dry size 6 for bed wetting and they fit him perfectly and he is pretty skinny.
Most 7 year olds are the same size in waist or at least near the same waist size unless they are more heavily built.
For the average 7 year old boy a size 6 or even size 7 diaper should fit just fine.
They also make a Pampers Cruisers size 7 or even Gentle Steps size 7 diaper that should fit the 7 year old boy if the Pampers Baby Dry size 6 won't fit good enough.
The diapers are actually better for bed wetting than pull ups or goodnites as they hold more and don't leak and are more comfortable for the kid to wear.