Please have him checked for an infected heart valve. 50% of horses have heart murmurs and its’t a problem at all. But they can pick up bacteria and get an infection in that valve, especially when they have dental work. And that is easily fixed. But untreated… It is damaging and deadly. In OTTB horses that are skinny it is usually a very good bet. Our current vet caught it in my mare after 5 other vets had checked and declared her healthy and just “lazy”. She is a drafty morgan and so didn’t look thin and therfore cause those vets to consider it. But, I knew something was wrong! It is an easy test. Lounging at a canter and checking for heart sounds while monitoring how quickly the rate of beats return to normal. I think it cost us $33? But it is diffcult to hear so you may need to repeat the test. Well, unless like my girl’s… It was a stage 3, we didn’t know if she would make it back. But antibotics for 30 days and she is fine. Just a word of caution. The vet said that raising the heart rate made pain in her chest. My girl tossed me on my head (yep, brain bleeding) out of the blue one day. She is Very sweet and would have never done that normally!!! So if your horse acts like he doesn’t want to work…. Listen.