When you are 13, your body start to change, but your mind too. For the ejaculation, it is normal : he will ejacluate soon. For the wet dreams, it is just that he is interesting for the first time on girls, but in a sexual way. Not every boys have this. Maybe he is in love with a girl and he found her very attractive. Some people can be very in love with someone without knowing it... I hope it helped you...
P.S : I'm french so maybe that I made so mistakeof writting or of comprehension. If I had some, just tell me...
At this point, you and your son should not worry too much about this. Most likely, he will probably be able to ejaculate soon, as the 13-14 age range is most common for a boy's first ejaculation. If another two or three years go by and he still has not ejaculated, I would see a doctor. As for the wet dreams, like the previous poster mentioned, not all boys have them. Hope this helps!
some boys get them, others don't. your son probably wont ejaculate for another year or so. it very in age. just tell him he's healthy, and to let his body unfold naturally.