We have had a few horses that cannot digest the Senior feeds, that seem to be high is sugar. We had a Appy/tb who was at the time 22 I believe that he was getting at least 3-4 lbs of senior and he wasn't looking any better. We put in on this stuff called Mormon's Senior Glo, which doesn't have sugars but its got rice bran and beet pulp, fatty acids, and very easily digested. You should look for something similar in your area.
We have never fed gumnuts or celery seeds. What does that do?
An easy no brainer exercise to do is put your horse in the bridle, and first teach him to lower his head by a gentle pull on his bit. Once he is soft to that, you can incorporate him moving his feet, while you holding the bit. If he puts his head up, gently touch it down. Don't make him keep moving his feet, wether its sideways or forwards, just get them started, lower his head, then start over again. This will teach him that he can move with a relaxed neck, and that he can move with it lowered. Do this both directions each way, and it'll help supplement the riding training.
Int he riding, try and teach him that when you squeeze with your legs, that he is to lower his head. To teach this, you just quietly and softly take an even feel of the reins, and squeeze with your legs. He may opt to put his head up, or toss it around, but keep everything the same until he can search for the correct answer. :) Little by little you'll be able to transfer that into all gaits.
The groundwork will help alot, I assure you. Some horses just don't know how to softly move their feet with their necks lowered, so you have to show him.
In riding, a horse's energy is like a river- guided by the banks but not stopped by them.