Seclude him during feeding time and give him good quality alfalfa, puts weight on just about any horse but don't give him to much, especially to soon. Since he doesnt sound particularily skinny is shouldn't take very much, maybe 2 or 3 good sized flakes spread out over a day (4 inches thick from a normal 70-100lb bale). Take him off the sweet feed and replace it with either a well balanced grain or senior feed. Sweet feed as not nutritional value. Off course, mix all this into his diet gradually.