Then increase fat levels with a quality fat, not the hydrogenated stabilized junk in the expensive high-fat feeds. Rice Bran Oil serves a few purposes in that regard. The fat helps stabilize glycemic response to other foods and forage, and the gamma oryzanol in it helps maintian a healthy gut microflora population.
As for adding "probiotics", it is always a good idea to do it about a week-10 days after worming or a course of antibiotics or a particularly high-stress incident, but I truly think folks who use probiotics on a daily, permantent basis are making a mistake. The goal should be to get the horse healthy enough that it can maintain a healthy population on it's own, without supplimentation.
There are many different types of microflora and enzymes in a horses digestive sytem, and these population adjust themselves for a horses specific diet. Dumping generic blends of probiotics regularly into a horse keeps the horses digestive 'gut critters' from "finding their own level" with regard to proper and efficient metabolization of feed and forage intake.
I consider dosing probiotic products as sort of "jump starting" the digestive system, like a car with a low battery after leaving the headlights on in a parking lot. When done occasionally as needed, it will have a benefit, you get to work. But if we jump start the car every time we start it, it'll create problems down the road and be detrimental in the long run.
Keep in mind I'm a bit ahead of the curve of most vets on seeing the benefits of Rice Bran Oil. Many just lump it in with other oils, without considering it's high level of Gamma Oryzanol. In most situations where rice bran is mentioned as a benefit, we would be much better off using Rice bran Oil instead. Aside from the fat and Gamma Oryzanol content, Rice bran is actually a pretty mediocre feed. We are much better off using a higher quality and more broadly benefitial feed, and simply adding the Rice oil which maintains all the qualities that set it apart from other fat sources in the first place. Good luck with your horse though. Post a pic, I love to see folks horses!