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.
Essentially, you want to feed him not only a low carb diet, but also a high fat diet. Fat is added through either oils or rice bran. The goal is to have 25% of your horses diet come from fat. I know that sounds scary, but PSSM/EPSM horses cannot utilize sugars/starches and so we need to give them an energy source through fat.
A good rule of thumb is 1 cup of oil for every 1000lbs of horse per day. (Flaxseed oil is best but Canola oil, Fish oil, etc. work as well)
Do not feed sweet feeds and look for grains that specifically say that they are low starch and high fat.
I personally feed beet pulp soaked in oil and dressed with a multi-vitamin supplement. Some people also feed alfalfa soaked in oil.
There are quite a few hard feeds that are high fat/low starch.
They all have a low starch/high fat grain.
Also remember, that sugars are in grass, and a PSSM/EPSM horse doesn't tolerate sugar/starches well, so a grazing muzzle or grazing after the morning is better... Do some research and also talk to your vet. Good luck.