Here are my 28 yr old Arab's before and after pics.
This first one was taken July, 2013. He'd had his first physical in June and the vet said not to worry about him ---- right.
I took the picture below this evening. Streeter looked like the above pic until a month ago when I took him off Nutrena Safe Choice Senior and put him on the Total Equine plus the Horse Lix tub.
I promise he was slicked out but we went from 89 degrees to 59 degrees in one day and he looks like he's got winter hair again
Streeter never has been a well-muscled horse, so I am amazed that his top line has come up as much as it has. The angle of the camera isn't skewing his top line --- it really has "risen" that much
Hope this gives you something to work toward with your fella. It's going to be a matter of finding the right combination of things for his metabolism.
I honestly would start with a vitamin/mineral lick tub in his stall. I am sure the bulk of Streeter's weight gain can be attributed to his lick tub.