Luvs size 6 should fit a 7 year old boy just fine.
My 7 year old boy who wears diapers for bed wetting used to wear the size 6 Luvs diapers until I found the Dollar General Gentle Steps size 7 diapers which are actually more absorbent and hold better than the Luvs diapers did.
I would recommend the Dollar General Gentle Steps size 7 diapers for your 7 year old boy or any kid that can fit into them and needs them for daytime wetting or bed wetting.
Pampers Cruisers Diapers size 7 should easily fit an average sized 7 year old boy.
If the 7 year old boy is of average weight and build then a size 7 Pampers Cruisers or any other size 7 diaper should easily fit him.
If the 7 year old boy is above average weight or overweight then he may need to wear a Youth sized diaper instead.
Another diaper brand that should fit a 7 year old boy are Gentle Steps size 7 diapers from Dollar General.
Or any other size 7 diaper should fit a 7 year old boy just fine.
The Gentle Steps Size 7 diapers are mostly plain white and are less babyish but very absorbent.
They are designed and made for older kids and younger kids that need diapers for incontinence or even bed wetting.