I agree with Susan. If he loves to play with his Boxer friend, I would let him. I'd keep a close watch, and be prepared to step in if I saw he was overdoing, but I believe, for my animals and myself, it is better to enjoy life, even if it hurts a little later, hell, even if it shortens things a bit, than sit at home safe and bored. This is something you have to decide, like deciding when it's time, it is a very personal choice, but that's my 2 cents