I'm not a Gamo fan but MartinT is so right. The BSA barrel is worth it alone. If you know how to chamfer channels, cut, recrown, the Stormrider might turn out ok.
Diana as a company is great, as are most of their products.
I don't know if you're limited monetarily or seeing if interest is even shown by your son. I can say this, in most things I've taken up, guns, guitars, archery. You're limited by your equipment up to a certain point. Air guns are much different than a guitar though.
I'm trying to say if you give him a junk trigger mismatched pellets, barrel that can't shoot ones slugged for it, he'll loose interest. This price point was around $5-600 with Marauders, Rainstorms. With a guitar, you get to solid quality & you're inspired. The "Gamo" BSA's that were mentioned are great rifles. Yeah, you may have to adjust trigger stages, break but it's a good solid PCP entry level.
Good luck