Do you know what kind of music your son listens to? you are pretty limited on your selection of subs due to price but music type would help a little on selction as well as on ported and or sealed.
Building a sealed box may not give the greatest overall output but it can generally get loud enough specially if this is the kids first system I'm sure he will be happy. Also by sealed he can keep more of his cargo area. But music type and maybe some dimensions and we could get a rough idea on a box for ya around some subs.
If you got some skills with wood working you should be all set doesn't take much, I find the carpeting is the most difficult. Thats the one place prefab comes in handy is carpet and such.