Alfalfa does really well for putting on weight. You just have to start slowly, with small amounts, and work your way up to a "full feeding." UC Davis did a study on emaciated horses using grass hay, grass hay with grain, and straight alfalfa hay. The horses getting the alfalfa hay put on more muscle weight with less stomach upset than the horses on the grass hay and grain diet.
We don't get bales of alfalfa out here, so I use Alfalfa pellets or chopped bagged pre-bloom alfalfa hay. With an underweight horse, I start with free choice grass hay and 1 lb of alfalfa twice a day. Mixed in with that I add a probiotic (like Fastrack or Probios powder), a small amount of a fat supplement (flax meal is my favorite, 1/4 cup to start), and a vit/min supplement (like GrandVite), no grain or feed. Over a two week period I work up to 3 lbs twice a day (about one 3qt feed scoop of pellets, or 2.5 scoops of the chopped hay) and 1 cup of flax twice a day. The horse is still on free choice grass hay, or a minimum of 1/2 a bale a day (25-30 lbs of hay).
I have put weight very well on more than a few horses with this type of diet. It works well on putting on quality muscle weight without making the horse hot/hyper or risk of founder, colic, or ulcers. I don't like using horse feeds or grains as most are full of fillers, byproducts, and "candy" that aren't good for a horse's digestive system. I feed a 100% grain-free diet. My horses get alfalfa pellets and/or hay mixed in with ADMs Staystrong Metabolic Mineral pellets, Cocosoya oil, and Apple Cider Vinegar. They are healthy and great looking horses with good feet and great attitudes.
I would have the horse's teeth checked & floated by your vet and follow your Ivermectin deworming with Quest Plus (moxidectin + praziquantel) in 6-8 weeks. That should clear him out completely and you can then use either rotational deworming or the fecal test method, whichever you prefer.