It sounds like he is reacting to a change in the herd dynamics. Now that his status is elevated to being the dominant horse, it seems he is challenging you for dominance as well. You need to re-assert yourself as the better horse and leader by re-establishing ground rules and manners. Pay attention to who's feet move who's. If he is forcing you to move out of his way or crossing your boundaries, he has you pegged as lower than him. Lots of groundwork and being absolutely firm about your space would help.
We purchased a mare that was bottom of the herd before we got her and as submissive as a newborn lamb. Since we got her home, she is alpha and continually challenges any horse and any body for that position. We cannot give her so much as an inch.