You may be able to see giant pacific mantas in Catalina. Another place for that is Bat Islands - it's either those or bull sharks. You might even see schooling rays!
As for the diving - it was not easy diving. In Cat we had a blue water safety stop, a lot of surge, big swells, strong current....all at the same time. It may have just been that day. I'm in the low 130's for dives and I think that one still stands out as the most difficult dive I've done. In Bats it was slightly easier but like Cats, no mooring lines so for descent or ascents. I don't think I would have enjoyed it or managed it with so few dives under my belt but it may be different for your son. Only the two of you can make that assessment.
I dove with Rocket Frog Divers. The owner, Michael, is amazing and their boats are extremely clean and very fast so you can get there and back in minimal time compared to slower boats. It's quite a distance to cover!