Yes Gentle Steps size 7 diapers will fit a 12 year old boy just fine.
I've seen some adults with a 32 inch waist easily fit into a Gentle Steps size 7 diapers.
I wear Gentle Steps size 7 diapers sometimes myself and they fit great.
The Gentle Steps size 7 diapers are cheap but good quality diapers that can easily fit most skinny adults, teens and young kids.
Gentle Steps diapers are mostly plain white diapers without any designs except on the waistband where the tabs stick.
But that is okay and they do absorb really good.