The USMLE is mandatory for applying to most competitive civ residencies. I'd be pretty surprised if there is absolutely no disadvantage to not taking it when applying to the competitive military ones. Although the military is pretty friendly toward DO's, there are definitley still some PD's who are biased, especially in the competitive fields. They're not going to care what your test scores are unless you have a usmle score so that you can be stacked up against the allopathic grads.
There is absolutely no benefit to taking the USMLE if the applicant is applying only to military programs. These "biased PDs" that you know just don't have a say in the way the point system is structured and, frankly, don't seem to have any trouble taking DO's into the most competitive military specialty slots. Also, he wants Internal Medicine, so he has basically a 100% of selecting for the residency of his choice at a military program.
A couple more thoughts for the OP. If you want a competitive IM fellowship as a DO (Cards, GI, Allergy, H/O), you would be wise to consider staying in to do it. There remains a significant bias against DO's in civilian fellowship matching. For example, in 2006 GI took 4x as many foreign allopathic grads as osteopathic grads (without the denominator, these numbers are hard to interpret, but I suspect that quite a few more than 14 DO's applied for GI that year). You also get paid as IM staff while a fellow which is a pretty sweet deal (120K/year while still in training).
If you are dead-set on applying for fellowship after leaving the military, I suppose you might consider completing the USMLE now to make that application more competitive. Good luck.