An average size 6 year old boy should be able to easily fit into Size 7 diapers especially if the tabs on the diapers stretch.
My 7 year old son wears size 7 diapers for bed wetting and they fit him just fine.
He wears size 7 parents choice diapers at night and they have plenty of room for him to grow into so size 7 diapers should fit a 6 year old child easily.
We tried bed wetting underpants such as Goodnites but they leaked and cost more so I convinced him to wear the diapers.
I used to wet the bed as a kid and wished I could have had diapers to wear because I would feel much better.
Waking up in soaking wet clothes and in a soaked bed full of pee was never fun for me.