It is okay to put a 7 year old boy in Luvs size 8 diapers or any other diapers during the day for wetting his pants.
Although if your 7 year old son is wetting his pants suddenly and it's continuing to happen and not just a few accidents you should have your son seen by his pediatrician to be sure nothing is going on.
But for now the Luvs size 8 diapers or any other diapers that fit your 7 year old son would be a good idea to help keep him clean and dry until he stops wetting his pants.
Here's a video of a 7 year old boy named Nathan wearing Luvs diapers under his Pajamas so Luvs size 8 should fit your 7 year old son just fine.
7 year old boy Nathan wearing Luvs diapers video