Did the vets check his teeth when they came to see him? Sweet feed is one reason he has runny poo. Grains don't digest well in the hindgut. Giving him fresh grass is great. As long as he's worked up to the amount in the pasture I wouldn't worry about him foundering or anything. Once he's built up though I would keep him out 24/7. You could try giving him some alfalfa pellets or cubes or some rice bran to help his weight. Probiotics are also a good idea as Beauseant mentioned. I give my horse SmartOmega3 once a day. It's a supplement from SmartPak.com. It's got prebiotics, probiotics, and enzymes for digestion along with flax seed and fish oil for omega 3s, which can helps against inflammation. You want to make sure if you do decide to give a supplement it has more omega 3s than 6s as the 6s can promote inflammation.