What makes you suspect he might have PSSM? I'm going through this right now with my mare, so I may be able to assist!
This week (we're having a hard time finding something she'll eat consistently!), my mare is eating 3.5 pounds of rice bran daily split between two meals and a cup of Cool Calories. She refuses to eat oil, so CC is a great substitute. I'm currently looking for a vitamin supplement.
The easiest PSSM diet is 2 pounds of alfalfa pellets and one cup of vegetable oil. Feed this twice daily, along with hay and a vitamin supplement. I tried this one with my mare at first, but she refused to eat it after a couple days. Picky, picky.
Here's a lovely table of options:
Your horse may need supplemental selenium. Make a vet appointment. Selenium is toxic in large quantities and you don't want to supplement unless you need it. The vet can easily test and see in what areas he may be deficient.
Seriously. See the vet. You can get him genetically tested to confirm PSSM. The vet can run a simple blood panel and check muscle enzymes a couple hours after exercise for signs of tying up. It's never good to suspect PSSM and not know