I would not recommend senior feed for a horse this age/condition. I really do not think he needs a lot of "groceries" per say, but definitely some real WORK! And a horse on a regular working schedule may need a bit of grain as a part of his diet to gather energy from.
He is long in the back, and a little straight in the pasterns, and his neck is huge, but I REALLY like how he looks. I would start riding him, a lot. Build his top line, promote self carriage and you will be amazed at how he changes. A neck sweat could work well in your favor too.
But as a performance horse, I would upkeep him on a grain.
Really nice looking horse!