I agree, introduce him to the grass slowly. Let him graze for 30 minutes at a time, 2-3 times a day. Slowly increase his grazing time over a period of 2 weeks. If you can't restrict his grazing time, then I'd invest in a grazing muzzle.
For feed, I would go with a nutrient dense feed like Purina Enrich 32 or Triple Crown's 30% Supplement. These feeds are meant to be fed only 1-2 lbs a day for full nutrition. Work him up to the full amount over the two weeks that you get him used to the pasture.
Add to the feed a fat supplement. Ask your feed store about whole flax, stabilized rice bran, or Nutrena Empower. Feed up to 2 cups of whole flax, 2 lbs of rice bran, or whatever the bag recommends for Empower. If your horse isn't gaining weight on the good pasture and small amount of feed with a fat supplement, you can add whole oats or alfalfa pellets to his diet for extra calories.
If your feed store is like mine and doesn't carry the best stuff (I really don't care for Purina's Enrich that much), then you can do mail-order from Uckele. Mix their Equi-Base Grass and Equi Omega Complex (2 scoops each) in with a 2qt feed scoop of plain whole oats. Uckele Equine Nutrition
- Equi-Base Grass Uckele Equine Nutrition
- Equi-Omega Complex
The Equi-Base Grass provides all the nutrients and minerals your horse needs at optimum levels. The Equi-Omega Complex is high in fat, fiber, and pro/pre-biotics for weight gain, hoof health, and digestive health all in one. Again, add more Oats or Alfalfa pellets if he needs more calories.
As the pasture starts to die off, bring in hay. Grass hay is easiest on their system. Go with some kind of Bermuda or Timothy. Continue with the feeding plan you're on. Hay does the best job of keeping a horse's weight up and the horse warm during the winter.
I recommend a nutrient dense feed (ration balancer) or the supplements with plain oats over a commercial feed because many of the ingrediants in commercial feeds are proving to be bad for our horses. My gelding was thin when we got him. It took a LOT of food on traditional feeds to put a small amount of weight on and keep it there. Once I took him off all that feed, he actually gained weight and kept it on with 1/3rd of the food (by weight) that I was feeding him before (as far as the grain & supplements). Ingrediants like corn, molasses, grain sweepings, and wheat can cause colic, founder/laminities, and negative metabolic changes (which can lead to negative behavioral changes). Both of my horses became healthier and happier once they were off the commercial feeds.
Good luck with your new boy and good for you for giving him a good home!