I have to agree with 'Old Shep', he may have been a sweet, lovable little bundle as a lamb, but things have changed.
This is where I find horns are great handles. When he tries to butt, sidestep, grab the horns, and flip him onto his back as though you were about to shear him.
He has to learn that you are 'top dog' not him. It may sound harsh, but if you want to keep him, you have to impress upon him that you will not stand for it. The only other solution is the slaughterhouse.