A 6 year old boy can wear size 7 Gentle steps diapers.
As long as your 6 year old boy is within 60 lbs of weight or less then they can fit into a size 7 gentle steps diaper.
Size 7 Gentle Steps diapers are made to fit kids from 41 lbs to 60 lbs.
A size 8 diaper such as size 8 Pampers Swaddlers are made to fit up to around 65 lbs which can be as big as a 10 year old.
A size 7 diaper including size 7 gentle steps diapers are able to fit most kids from 5 years to as old as 10 and sometimes 13 depending on their waist size.
My 6 year old son who wets the bed wears a Gentle steps size 7 diaper and they fit him perfectly.
I would say that he could wear the size 7 gentle steps diapers until around 12 to 13 if he needs them that long.