I agree with checking your horse for pain first. Often when horses have been in an abusive situation with another horse who has been the alpha, as I suspect with your horse if he was with a stallion. Many times the horses have a lot of tension within and that needs to be released. This can be done easily by doing some Tellington Jones, message, or My favorite, Myofacial release methods. Actually I would start with Mane pulls all along the top line and hold each for three minutes or until your horse starts releasing by chewing or yawning. This often is enough to relax the horse and start becoming your partner.
I also wonder how much ground work you have done with this horse previous to mounting him. . This horse must respect what youare asking or I would work on this. This is all done in a gentle method, all this work is done from the ground and if it isn't, I sure don't want to mount the horse.I like to have my horse staying off me and I do this by working his eye off me. He should turn away from me on the forhand. Also work the horse to move his hind away from you. These are things that should be securely in place before mounting.
I also wonder how he was treated when you were away, but I almost wonder if your horse was allowed to show attitude as he decided to be the Alpha, and if you are affraid he is sensing that. Take back your possition as the one who is in control. I would start by sending him away when he shows aggression. There is a time to baby, but not when agression is in the picture. Do not reward your horse for bad behavior.