A 7 year old boy should be able to easily fit into and wear a size 7 Pampers Cruisers diaper.
My nephew who is 7 years old wears Pampers Cruisers size 7 for bed wetting and on long road trips or when away from home.
He sometimes has bladder issues and wets his pants when away from the house so he wears the Pampers Cruisers size 7 for protection.
Some 7 year old boys may also fit into a Luvs size 6 diaper or Pampers size 6 diaper.
Pampers and Luvs have pretty stretchy tabs so they can easily fit most kids 12 and under.
It all depends on the build of the kid as some kids are larger than others but most average size 7 year olds or even 8 year old boys should be able to fit in and wear a size 6 Pampers, Luvs or a size 7 Pampers or Luvs diapers.
There's also another diaper called Gentle Steps diapers that come in a size 7.
These Dollar General Gentle Steps size 7 diapers can fit most kids up to 10 to 12 years old and so a 7 year old or even 8 year old or younger should be able to fit into the size 7 diapers.