Horse might have ulcers?

My Thoroughbred gelding who has always been a pretty easy keeper and eaten anything you throw at him has been off his feed lately. I noticed a couple of weeks ago that he is looking a little thin but we have had trouble getting good hay lately and the feed from my supplier has been sub-par. I have switched his feed and now he's getting better quality 12% pellets and some (expensive) but good quality hay. The trouble is that now he doesn't want to eat the new pellet feed but he is chowing down on the hay. He has no colic or virus symptoms and is normal in every other way. I am worried about him because he is losing weight. He is on a weight builder supplement but since he's not eating his grain, it is doing no good. He never had ulcers on the track when he was running but I am wondering if he could have recently developed them. He's in moderate work and not acting stressed out. Anyone have thoughts on this or experiences with ulcers in their horse?