My horse was SUPER skinny when I first got him, it was awful. He was about 100lbs underweight too. He came from a show barn, so I assumed it wasn't worms but I did the worming booster just in case. I fed him Strategy and a fat supplement for about 9 months and he did just fine on it. He gained a good amount of weight, but he never really filled out though.
Eventually, I started looking for something better. A very helpful employee at my local feed store educated me on different grains and ingredients. With her help, I decided to switch him over to Tribute Kalm Ultra and I love it! He's been on it for about 6 months now and he looks fantastic. I only have to feed him 1lb AM and PM, before he was eating 3lbs of Strategy AM and PM.
Kalm Ultra has less sugar which is better for all kinds of health reasons, and it has a higher fat content. Plus, it's got probiotics, omega 3 and 6, and a whole bunch of other vitamins and minerals. It's about $17 per bag around here, but it's worth every penny. It's definitely worth a try, such a great quality feed.
Kalm UltraŽ Horse Feed | Tribute Equine Nutrition
Just a suggestion! It's ultimately up to you. I really recommend learning about what different ingredients do for your horse, it's fascinating and it might change the way you look at grains all together. They are more than just tummy fillers!