I would definitely switch the feed. Just because it says "Senior" does not make it the best food for your horse!
I also feed tall fescue hay and I find it is great for my competition horse because of the high energy content without too much sugary alfalfa. Maybe try to mix in some orchard grass hay with this hay.
For the supplementing, try to see if he can sustain himself on mostly hay. Get a knowledgable horse person to help you rank him on the body score scale (Henneke Body Scoring - Habitat for Horses - Equine Protection Organization - Horse Rescue, Rehabilitation, Horse Education and Adoptions
If he is already a good weight (5 or 6) then gradually switch him over to beet pulp and rice bran and give him access to loose minerals or a mineral block.
I suspect he is most likely over weight, in this case as you are switching over his feed, reduce the volume until his weight stabilizes.
Hopefully the changes in the feed and keeping him from becoming overweight will help some of the behavior issues.
And in addition I also suggest finding someone to help you out with him.