1. If your horse is hot or nervous, don't feed senior feed, sweet feed, or alfalfa. He'll just burn off any potential calories.
2. He needs more grain, around 6-8 pounds a day, and he cannot be fed that all at once. You have to take into consideration that a horse's stomach can only hold 5-8 pounds, and if you give him over that, it will be pushed into the intestine and will not be absorbed.
3. Beat pulp. Not handfuls, quarts.
4. It honestly doesn't sound like he's getting a lot of food, but I'm not sure what lankin lights are.
5. If you need to put a lot of weight on in a short period of time (like for a show or something), feed him three times a day. I had to do this with my show gelding after he came out of the hospital, and he put on a phenomenal amount of weight.
6. Add oil to his feed. A lot of people will say corn oil is fine, but for putting on weight I like buying the oils specifically for horses, with Vit. and Min. added in them and stuff.
7. All the hay he wants.
It might cost a little bit more, but you'll find that once you get him to a healthy weight, it will be much easier to keep him there. When buying a feed, I like pelleted feed because it is easier for a horse to digest. Also, don't get anything too high in protein (12 is MORE then enough). Your horse isn't going to use it, he'll just waste all that extra money.