Why is my husband having frequent wet dreams?

Let me provide some detail: we are a young couple, he's only 20. This has only started the past year, he said before that, he has probably only had two wet dreams ever, in adolescence. Suddenly though it's become an annoying and unexplainable problem, they occur every few weeks or months, sometimes several nights in a row. I want to point out, because I don't know if it makes a difference, that these are never full ejaculations that he waked up to, just lots of "pre-***" (sorry to be so frank, I don't know the medical terminology for it!). Lastly, he swears that he either never remembers what he dreamed about, or that sometimes his dreams are even anti-sexual. For example it happened this morning, and he says he remembers dreaming about a video game. So apparently these wet dreams can NOT have to do directly with sexual stimulation? Someone who has experience or knows the scientific reasoning behind this, please explain, because it's so hard to find this info everywhere, and he's too embarrassed to go to the doctor yet. Some of the questions I'd like to find answers to are: is this a sign of sexual disorder? What is going on to cause these to happen frequently? And ARE the dreams sexually based or not? (in other words, is he probably not being honest about his dreams being sexual?