Senior feed, start with one handful at a time. Not one 6'4" logger type handful but a woman sized handful and increase one handful at a time. A good name senior feed will be easy to digest and have probiotics in it. Get him through a week and add a little multi vitamin or ration balancer to it. At the amounts intended for a small pony at first. My 37 yr old, 11 hand pony gets 1/3 of a cup of ration balancer at each meal. The rest is mostly hay with some beet pulp on single digit or snow storm days. Slow, slow slow! His gut is severely compromised at this point. Any concentrated feed needs to be just a taste or two. I can't stress this enough. More and you risk colic and/or founder.
How old is the horse? If he is having trouble chewing hay you can soak some beet pulp and hay cubes and make him a mash. Careful there too because as a mash he can consume a lot more faster than chewing stemmy hay. Even if he can't chew very well let him have regular hay anyway. They need it for their psyche.
Worry about weight building things in mid Jan.
Keep colic and founder first aid supplies on hand.