My boyfriend is a US Army Veteran and has been diagnosed with PTSD and depression. His psychologists send him a ton of pills every month-sertaline, cycolobenzaprine, mirtazipine... the list goes on. He refuses to take them because frankly they make him worse. He becomes either moody and irritable or just plain numb and he'd prefer to live with his insomnia. My question is this: I've noticed that on weeks when we have frequent sex, his overall mood is better and he just seems all around happier. I guess I just want to know if this it normal for men in general to have that marked a change in attitude or if his mood swings and depression make this change more pronounced. Is it possible that a nookie session a day could keep the nightmares away?

I think you are onto something here. I found this, knowing that serotonin is a feel-good chemical and then throwing into my search the word "sex" --

Serotonin
What it does: Serotonin regulates your moods. Having an orgasm releases an extra shot of serotonin to your brain, which acts as an anti-depressant.

How it makes you feel: You feel cheerful, hopeful, emotionally balanced, and content. Most clinical anti-depression medications pump up serotonin levels in your brain.

It's the natural high
Some people go to great lengths to get drugs, but why bother when these active chemicals are just waiting to be released in your body? Having an orgasm is like getting high. By releasing a myriad of substances into your bloodstream, you are altering your brain and body functions.

Orgasms involve a complex interaction between three systems of the body: the vascular system, the nervous system, and the endocrine system. They are so complicated that it's a miracle that they happen at all. So enjoy them and remember to have as many as possible.

Read more: Chemicals That Fuel Your Sex Life - AskMen

It might make sense for some. Not all PTSD is the same.
Pleasant sex raises serotonin levels, and serotonin raising elements are in a lot of mood stabilizing drugs. Relaxation too, why not. It's exercise, and exercise is good for stress, depression, health in general.
So I guess you are helping him. Just don't count on it being enough.
There are new studies of vets with PTSD that show that blood pressure lowering drugs, nothing fancy, nothing psychiatric, work to stop flashbacks and associated violent outbursts. The trouble is that they need to be taken just before an attack is felt coming on, not all the time. So I haven't heard much about this very hopeful work in a few years.
He should really consider going to group, if there is one near him - there should be. There are so many vets with PTSD and the suicide rate is very high.

Edit: I was writing while Wondergirl posted.

Joy is correct is that sex probably will not be the total cure for him. Exercise, individual counseling, and a support group will also be to his benefit. Staying on top of each episode and looking at the forest as well as the trees will help him live a healthy and fulfilling life.

He has been on the blood pressure medications and reported that they made him feel foggy and drunk and he operates heavy equipment so he discontinued them. And he doesn't have flashbacks or outbursts, per se. Normally he jut has severe nightmares and any flashbacks are brought on by some sort of stimuli, i.e. loud banging noises, someone goofing off jumping on his back or sneaking up on him. He is required to attend counseling at the VA as part of his VA disability-he also broke his back, lost most of his hearing, suffered traumatic brain injury, and injuries that impair his fertility. However, he is a closed book when it comes to his emotions in most cases, and especially in this one.

The BP drugs should only be taken just before the stimuli that are going to set him off! Of course they make him feel foggy. How about before sleep (and after sex)?

I hope the counseling is a group. I see no value in one-on-one. He needs to talk with others who have been through this, with a group facilitator.

The brain injury might be the key to his closed emotions, and who knows what else. What does he think about that? And you? Have you researched the area of his brain that was injured?