It is behavior that shouldn't be encouraged. Some of it could be in play, but it could also be a way for him to try and establish himself. While he is a only wether (yes, you got it right) all goats establish a "herd' hierarchy even if the herd only consists of 1 ot 2.
He is displaying a typical goat behavior, however it is towards humans.... and if he ever followed through with a hit, it would hurt as well as could do damage, horns or not.
And, if he thinks it's ok to do this towards humans, he will think it's ok to do this to all humans no matter their age, size or frailty. The more he gets away with it, the more it could encourage him to do it/get worse.
A couple of things you could do to teach him that it's not ok, it to carry a little squirt gun with you and squirt him on the face when he displays this behavior. Most goats hate getting wet and this is a good "No" without having to make any physical contact, which he could interpret as return "play."
A few of my does have done this once or twice and I'll usually loudly clap my hands in their face and say No. While this usually works on the girls, boys can sometimes be more persistent, so maybe try that first and if it doesn't work get a squirt gun.
A shaker can also works. Like you might make for a dog..... fill an empty soda can with 10 pennies, handful of small pebbles, some bb's or whatever and tape it shut.
When he rears up at you, give it a good hard shake and say No.
Like the squirt gun, it's the shock value that you are going for... to startle him.
He will soon learn it is not desirable to test the humans.