It's tempting to make assertions like "men NEED to ejaculate for prostate health," but, like most aspects of sexuality, nothing is universal.
So let's talk about what's possibly happening with your boyfriend. Of course, without hearing from him, everything I say is speculation.
Orgasm affects people differently: some of us are energized by it, and others enervated. Some people jump right up and run off shopping, while others lie around in bed napping. As in most things sexual, everyone is different.
Sounds to me like your boyfriend has convinced himself that ejaculation is tiring for him. If so, there's little you can do to change that, aside from referring him to an accurate book about male sexuality (more about that in a minute). Or he may have some old, unresolved fears about ejaculation.
I'm wondering why YOU think it's a problem? If it's not a problem for him, then why worry?
However, from what you describe, it sounds like his fatiqued state is related not to orgasm, but to depression and perhaps stress. If this fatique is affecting both his quality of life and your relationship, then suggesting that he get a thorough check-up might be a good idea. If he's clinically depressed, he may need to explore the option of a mild anti-depressive (be sure it's not one that affects erection/orgasm).
Now for that recommended book: "The New Male Sexuality," by Bernie Zilbergeld, Ph.D., widely available used and in paperback. This is the bible of male sexuality and contains a wealth of helpful, accurate information. Happy holidays to you both! Dr. J