I have a young neighbor kid that will walk in without knocking, and it is probably my fault that he does, but it does scare me. I do have a very big dog, and a bunch of little dogs, one that I know will bite if he pushes her. (He is clueless enough that I don't trust him) It has scared me more than once. I now have to give him a few more clues, like, that he has to call me before walking into the house. I also need to keep the the door locked too.
But No one should walk into your home without permission. Another option would be the next time the guy is expected to drop by, have the biggest meanest looking guy you know or can find, waiting for him to walk in. I bet he will never do it again, if he knows any better. Some guys tend to think that women alone are fair game, and enjoy making a woman without a man in the house uncomfortable. He may just need another guy to explain common courtesy to him. Of course there is always the option of simply meeting him with a shot gun. The stepson will probably never forget the lesson either.