Get a fecal count and then you'll know what to do in terms of the parasite load. We have had hard-keepers in the past (TB) and a 28 year old gelding that I have to stay vigilant about his weight. We don't over-grain (but he gets grain daily) and we feed regular grass hay. For supplements, I use flax seed oil and Cool Calories. Thus far it's done a good job of both putting on and keeping on weight, but it was a slow process 15 months ago when he got ill and dropped a couple hundred pounds. However, i'd prefer to put the weight back on slowly and then keep it on long-term.