he may just have an attitude, the horse I ride is 4, my owner has had her since she was 1 and right out of pasture, no history of abuse. She just has a plain attitude. Wwhen you tighten your girth, she doesnt snap, she just tosses her head. Maybe you should slowlt take the girth, let him sniff it, tell him that its not going to eat him, once he is calm with that, rub it all over his body. Once he is calm with that, attatch it to the saddle as loose as you possibly can. After he is comfortable with that, slowly and gradually tighten it. Do this as many times as you come to the stable for a lesson, or just to hang out. I only just thought of this, so im sorry if it doesnt work! As for his behavior with the other horses, I don't know how you could help that. Maybe take him to a horse that is IN its stall so really they can't kick eachother and let them get comfortable, maybe he'll make a friend or two =] hope I helped!