Marine Corps, here. Not infantry, although I did grunt shit after I go out for a few years, there are as many if not more differences as similarities. Shrug. What we did and where we were seems to matter less than I'd ever have thought. I belong to a combatPTSD site where it's pretty much all services, from all over the world, all conflicts, all over, from Nam to Present... And it's amazing to me how none of that shit matters. The more things changed the more they stay the same. If it's combat trauma, not just military trauma (whole lot of possible trauma just from serving, lol, the military can be a seriously f*cked up place), I'd recommend whole heartedly that you have your BF check out the sister-site mycombatPTSD.com
Least in my experience (first hand, and dating other combat vets) sex tends to split, or flip flop back and forth, between no effing way (for a helluva lot of reasons; body just isn't working right, mind can't focus, meds, just feeling sick as f*ck, etc.) & seeeeerious overdrive.
There are some really great articles around on combatPTSD & sex I'll see if I can dig out. One way or another it's often an issue.