Yes size 7 diapers should fit a 7 year old boy.
However it depends on the size and waist size of the 7 year old boy who is gonna be wearing the size 7 diapers.
If the 7 year old boy is a bit overweight and not average sized then they may not fit into the size 7 diapers.
If the 7 year old boy is average weight and has a small waist size then the 7 year old boy should easily fit into the size 7 diapers.
If the 7 year old boy needs to wear diapers and will not fit easily into a size 7 diaper then you can buy Youth disposable diapers online that will fit them.
My kids wet the bed and wore baby diapers until they no longer fit and once they no longer fit and needed diapers at night or on road trips for protection they wore some Attends Youth Diapers.