Luvs size 8 diapers will fit a 7 year old boy, especially if he's on the skinny side.
Even a size 7 should fit a 7 year old boy that is of normal weight and build, although if he's bigger build or weighs more than normal, then he may need larger Youth sized diapers.
But for most average sized 7 year old boys a size 7 or size 8 Luvs diaper or other size 7 or size 8 diaper should fit him just fine.
Here's a video of a boy named Nathan that is 7 years old and he's wearing a Luvs diaper and so while not sure what size he's actually wearing in the video, it at least shows that a 7 year old boy can wear a Luvs diaper or other toddler sized diaper in most cases.
https://vekdrive.com/drive/s/V7H9chdXo9JjbLUn09PwYIIFRpdL9U
When I was 7, I used to wear size 6 diapers for bed wetting and they fit just fine.