Size 7 baby diapers should easily fit most 7 year old boys.
Although it depends on the 7 year old's weight and waist size but for an average size and average weight 7 year old they should be able to fit into a size 7 diaper.
Although if the 7 year old is overweight or weighs more than normal or is built bigger then they likely won't fit.
In that case if the size 7 diapers won't fit the 7 year old boy you can buy youth diapers online.
My 7 year old nephew though is skinny and he can fit into a size 7 diaper just fine.
My nephew is a bed wetter and wears the size 7 diapers for bed wetting.
He was wearing pull ups such as Goodnites and they would leak on him.
So his parents bought him size 7 diapers which are the Gentle Steps size 7 diapers and they fit him perfectly.
He can even fit into a size 6 Pampers and size 6 Luvs diaper.
I know too that the size 7 Gentle Steps diapers run large and even Pampers Cruisers size 7 run a bit larger than Gentle Steps size 7 diapers.
But if your 7 year old is too big for the size 7 diapers then you can cheaply buy tape on youth disposable diapers such as Attends Youth Diapers that will fit your 7 year old just fine.