I don't know of any restrictions; however, in addition to the advice above to not do something he will later regret, I would add don't participate in any activity that could result in an injury. An injury could "nix" his appointment offer.
As a past parents Club president, I used to tell parents that when your DS/DD gets the appointment, wrap him/her in bubble wrap to avoid an injury. Obviously, nobody wraps anyone; however, the point is that after completing such a long and demanding application process, each successful applicant would be wise to minimize activities with injury potential. The risk of losing the appointment due to an avoidable accident is not worth it. Be careful and be prepared to start BCT.