The short answer? Nobody's taught them. Or women, either.
One of the big problems we have as a society is the fact that we're expected to just know how to do things like have a relationship. A lot of people assume that hormones will take over and do the job: my favorite piece of horribly wrong sex ed advice is from the '50s, which says if your daughter asks what being naughty is, to tell her she'll know when she gets caught.
If you don't have good role models, or people to ask, figuring out how dating and relationships work can be difficult on the best of days. The only way out of it is open communication: how's he going to know what he's doing wrong if nobody tells him?