1) Get the vet out to do a check, especially the teeth. There is NO way to go around this, especially with a rescued horse. Also have the vet do a fecal egg test for any worms in his poo.
2) Get him on a high fat/fiber diet. I recommend rice bran, beet pulp, and alfalfa a pellets if needed. Make sure you introduce the feed very slowly. You can kill a malnourished horse introducing a diet to an him/her fast. Also, have free choice grass hay, and plenty of room to move around, don't lock him in a stall, unless it is for quarantine reasons.
“Good things come to those who wait… greater things come to those who get off their ass and do anything to make it happen.” - Unknown