A "craphat" is anyone who's not done P Coy ie. if the beret's not maroon then it's crap, including green. It's all harmless banter anyway - they do different jobs so each to their own.
But anyway, back to the OP: make sure your son is clued up on the history of 3 Cdo Bde and their current structure and operational commitments. Also, make sure he knows why he wants to join the Bde instead of say, the Paras or "normal" infantry.
As for phys, he's still very young so doing free weights or tabbing with weight is more likely to cause problems than make him fit. As other people have said, rugby training is ideal as it also increases robustness and determination. If he wants to do extra, then press ups, sit ups, pull ups and running is ideal. Make sure he's getting the technique bang on as per the RM fitness tests as incorrectly executed exercises won't be counted if the PTI is doing his job properly. Email me if you want any more advice.
Oh, and good luck.