My horse is the exact same way. He's a 19yo OTTB whose high strung. I use a mix of Tribute Kalm Ultra and Tribute Kalm N Easy, no supplements needed. I feed two scoops per day plus hay two times a day plus occasionally goes out on grass at night. This seemed to work for him. He put on 58lbs in about 5 weeks times. Plus he's been in work.
I was just curious about brands of senior feed you guys have had success with. My husband is a horse vet-he gets all medical items done regularly including teeth, shots, working based on fecal tests, etc. I have had him since he was 3 & he is 26 now. Just seems to be getting more difficult keeping weight on him. He currently gets grass, alfalfa hay, rice bran & purina senior. He previously got a if this & triple crown senior but the consistency of the feed seemed to changed toward the end of last year & he didn't like it as much, so I switched. I was thinking of trying the purina equine senior active since he eats hay.
Rice bran is a personal favorite of mine, as well as beet pulp. I prefer rice bran though- and I wouldn't do a daily worker because of how resistant parasites are becoming, but fecal testing every 2-3 months would be a good idea.