Pampers size 7 diapers will fit a 6 year old boy as long as the 6 year old boy is skinny.
My son wore Pampers size 7 diapers when he was 6 and even until 8 years old when he wet the bed and on road trips because he would fall asleep in the car and wet his pants.
So I convinced him to wear the diapers for both bed wetting and during car trips.
He had a hard time holding his bladder when we could not make it to a bathroom at a rest stop in time and he's wet his pants before on road trips so he wore diapers on road trips just in case.
The diapers made it much better for him and I told him that if he needed to pee to go ahead and then change the diaper later when we could stop.
Holding your pee in is not very good for your bladder, kidneys etc.
And sometimes on road trips there could be 50 to 100 miles or even sometimes longer before you find a bathroom to go pee.