I have a question. Have you or did you ever play with him like by pushing on his head or letting him butt your hand? Things that let them learn butting a human is OK, even in play, helps teach them poor manners. It is adorable when they are tiny...not so adorable when they are big. I actually told off my father when I saw him playing a heatbutting game with my Nigerian Dwarf buckling. BIG NO NO HERE. I do not ever want him to think headbutting a human is OK. The few times it happened already made the buckling think it is fun and fine to do, so I've been working hard to get him to stop. His behavior likely turned very sour after you removed him from the does for a reason: goats are herd animals. Bucks need buddies, too. I have two bucks. They romp, heatbutt, and play with each other. Before I had two bucks, I had a buck and wether. They'd do the same thing. He is probably all spun up from being in rut, stressed that he is suddenly alone, and sees a human as a convenient target to take out his frustrations. Give him a friend, and an enriching enclosure with toys and things to jump on, and he'll likely settle down. In the mean time, keeping your niece away from him would be wise.