Just a couple of thoughts, though they're probably worth what you pay for them:
1. Is there any chance there is an active bone infection? If so, another antibiotic might be more effective. My clinic tends to use Clindamycin for bone stuff.
2. I realize that you don't want to torture him, but if you're like me, you're going to drive yourself nuts if you don't have him x-rayed, especially if he limps a bit. Having said that, if it is OS, it's going to grow, and you'll see it.