Oh man, tricky situation because telling the exact truth, especially such a simple truth based in something psychologically complex, could easily be perceived by overly cautious friends/family as a cause for alarm, that maybe you're hiding something.

If you're just talking about acquaintances that's a different story, and you should just say whatever. "Eh, my nieces and nephews play a little rough". "I work out with weight bands" "I spent every Friday in prison a prison cell as performance art" :)

For closer people, I think you either have to have a believable alternative, or you have to spend some time planning a few simple ways to phrase the truth that the people you know will understand.

The only thing I could think of would be to phrase it in such a way to sound as though you are telling them something in confidence because you don't want to embarrass him, but that you're only telling them so they know not to worry. Something like:

"Please don't repeat this okay? Have you heard of PTSD? Well, sometimes at night it causes my s.o. to have really intense scary dreams and I told him he doesn't ever have to go through that alone, and just grab my hand/arm and I'll hold him tight. Some nights he's so scared and shaking and that he holds on a little tighter than he realizes. The doctors all say this is a normal part of the process for people dealing with PTSD and he's on the right path and getting all the right meds, but it's just going to take a little time to work through it."

Obviously just an example, but I think spending some time customizing a general response that is both honest, protects your bf's feelings, but puts this situation in a sympathetic light where they feel more compassion towards him and less afraid for you it key.

Tricky situation, and I think your responses will really have to depend on the type of people asking you about it.