I prefer for him to not break bones. If an activity is 50% likely to involve bone-breaking, I'll step in. But if that really is the worst that could happen, and is unlikely to do so, then I'll try not to.
I take comfort in knowing that humans have been raising humans for a million years, and it usually works out alright. That must mean our own instincts are mostly good enough.
Also want to point out what another commenter mentioned; technically anything that can break a bone can do more damage than that.