I decided to say no and already told him. Basically because I think it makes sense that there is not enough time to train, that I don't know the coaches would put the needed kind of thought into the training (and I am not going to take that on), that I think there is a lot of potential for injury given these other factors, and that ds does have a lot of other great things going on to focus on.
Thanks so much for all of your great input!
lucky single mom to 20 yr old dd and 17 yr old ds through 2 very different adoption routes