What type of salt or mineral block do you recommend?

A few things to consider would be having his teeth floated and getting him wormed if you haven't already. You can feed the Equine Senior in higher quantities and cut back on the alfalfa. The Senior is high in fiber, low in starch and can even be used as a complete replacement for hay for horses that cannot eat it. I would consider giving him 3 feeds per day - scoop and a half at each meal and then just keep good quality gr hay in front of him - you don't what to founder him. Make sure he's blanketed if it's really cold so he's not wasting calories on keeping warm and just any old mineral block will do. Salt and minerals are just that, no need for a fancy brand. I get them from Tractor Supply for $1.99 and break them in half. I have a horse on 4 months stall rest on Equine Senior and my neighbor who owns a feed mill put his filly on it following her surgery. It's good feed that keeps condition on them, is non heating and because you can feed it in greater volume, it's good for getting weight on.