If you can find one, a Beretta Urika 391 is probably the best semi-auto. Semi autos will have softer recoil than O/U, so depending on your son's size, that could be a factor.
If it's trap he likes specifically, then get a trap gun (Monte Carlo stock, mid and end beads, high comb). If he wants to shoot all kinds of clays, a Sporting gun is probably more flexible.
The 391 series is a discontinued line at this point, but can still be bought, mostly used.
The current Beretta semi-auto series, the A400 series, does not yet have a trap version, but they have a sporting that you can buy in Canada, and they have a parallel target (more like trap) version that will be available in Canada soon enough.
I'll leave it to others to comment about O/U, since I only use semi-autos.