Tips To Feeding An Underweight TB?? Please Help!?

Before I bought my OTTB a few years ago (( 2006 )) he had been raced and abused. So when I got him he was very underweight. At the time I was at an eventing barn, and the owners liked their horses a little on the thin side, so he didn't gain much weight there. Then in summer of 07 we changed barns. He gained weight, and his overall condition improved greatly...until wintertime. Due to being in school, no indoor, and very short days, he lost some of his weight. (( even with blankets and added feed )) So this year he has been on a training schedule and he gained his weight and conditioning back. Of course something else had to go wrong; His like several other horses' dewormer stopped working. He then developed rain rot and lost most of his weight and conditioning. The rain rot is now gone and we're working on having him gain weight. We've tried everything from beet pulp to senior feed. He's feed twice a day, (( 4 flakes of hay, and he's had up to 8pounds of gain per feeding )) though he still hasn't gained much weight! Could he just be a very lightweight/small framed Thoroughbred?? Any help would be great, as winter is coming up! Thanks in advance :]