I agree that 5 months is too young to start hunting but my dog was 4 months old when he and I would go out shooting ground squirrels and he had a ball. When bird season rolled around a few months later he was ready to go and give it a try.

On the second question, I wouldn't worry about it at this time. Let him bulk up then get use to swimming and retrieving in the water and then slowly work your way up to jumping off a pier or raft or something.