Has he been checked for ulcers? Teeth?
Free choice hay is good. A couple of flakes of alfalfa will help, not only for weight, but also to build muscle. I wouldn't give senior feed. Most of them are formulated as complete feeds for horses which are not able to eat hay, so they're relatively low calorie, most of them just slightly higher than hay. I'm also not a great friend of CC...dehydrated vegetable fat. I'd pick a feed with at least 6% fat, 8% being better, and NSC 20% or lower. First ingredient not being grain, if at all possible. I'd also add 4-6 oz ground flax, for extra fat and the good omega's, since he's only grazing very little.
I'd start out with the minimum recommended amount of the feed, plus 29
0%, and give it about a month, taking pictures now and in a month.