Maybe it's time to introduce the concept of doing a proof. I know proofs are out of fashion in high school geometry these days, but they are very important past calculus. Math is all about showing how you got the answer.
Of course, if he's just doing simple addition, he can't really do proofs for those (until he takes number theory...)
In terms of alternative methods, if the point of the exercise is to show how different methods produce the same answer (thus getting a much deeper understanding of the number system), then it is important to do the problems different ways. If the point is just to get the answer, then it shouldn't matter how he solves it so long as the solutions are consistent.
