Acetaminophen works by a different pathway and doesn't affect bleeding time. I am not sure I would fret too much either way, since it is highly unlikely that major bleeding is occurring.

With it not being a compound fracture, infections are not a worry. If he is stable and not experiencing severe internal bleeding (again, major bleeding is unlikely), he should be fine for 12 hours. Pain tolerance will be an issue, but a sling can be bought or made out of a triangle bandage. There can be a lot of discomfort even with a sling, but padding with a pillow between the arm and the chest (and even securing the pillow and arm in place further with wrap or ace bandaging) can be helpful.