I board so have no control over the hay my horse is fed. It's not bad but does vary in quality. He's a little overweight but mostly just has kind of a bloated hay belly. He's all UTD on worming. I'm wondering if a pre- or probiotic would help the situation.

Its worth a try I would think..

What are you feeding?   Nevertooold (Forco dealer) isn't able to get on here right now but told me to ask that!

This is all from her:  Forco can help with hay belly. It enables digestive system to work at best of its ability and allow food to be digested and nutrients to be absorbed into horse's system.   Lack of good quality hay and feed will cause hay belly or lack of digestion system to be able to break down the nutrients.   

I board so hay is inconsistent. Sometimes it's really nice other times it's meh. Grass with sometimes a bit of alfalfa mixed in. He's on a slow feed round bale 24/7. He is a bit IR so gets just 1/4 cup TC Lite mixed in a little bit of soaked beet pulp and alfalfa cubes, all of which are just delivery mechanisms for his supplements. 

I would have your horse checked over by a vet to eliminate anything else.

The newest research is claiming a prebiotic can help. Besides his hay belly, does he have fat deposits on his neck or tail head?

Some of the newer research is also connecting stress with IR and these horses should have hay in front of them all the time.

If he was my horse, I would talk with my vet  and rule out any other problems. 

How long has he had this problem?

How long has he been in his current boarding situation?

What is your worming program? 

All the above is good advice. FORCO is a very good product. It can be a real help for diets that have a base of lower quality hay. Also check teeth. A horse on poor quality roughage that can not chew comfortably may exibit the effect that you are seeing due to poor processing of the roughage in the hind gut.