:yeahthat:
If the reason for his low weight isn't known, make sure he's checked for parasites. I just work them up to what they should be eating (I feed unlimited hay, small amount of pellets, and a variety of veggies), maybe add slightly more, then reassess after a few weeks.
It seems counter intuitive, but exercise also helps. Start with short exercise times, especially if he seems to get tired. A lot of those skinny rescue buns have no muscles to speak of which just makes them feel more bony. Allowing them to exercise helps them build muscle instead of fat and will increase their overall health.