You don't know if he spent any time on TPN while waiting to heal. TPN, a nutritional IV, is not all that uncommon in trauma.
Also, getting shot by FMJ really isn't all that bad. These types are never smart enough to realize they'd be far more effective if they just spent the extra money on a quality round. Ballistic gel demos give you an idea why FMJ is far easier to manage if you aren't familiar. That temporary wound cavity is not to be fucked with.