When I researched ulcers in horses some of the other symptoms were poor appetite, poor hair coat, mild colic, attitude changes such as a calm horse suddenly acting hyper or different, poor performance, lying down more than normal, sensitivity to touch on their lower stomach. He is not exhibiting any of these signs. Other than the weight loss he is completely normal. His coat is shining and looks great, he hasn't been lying down any more than normal and he is great with being rubbed all over. I know this doesn't mean that he doesn't have an ulcer I am just inclined to believe that it could be something else as well since the weight loss is the only sign I have to go off of right now.
I too am interested to find out what is really going on. I just heard from his leaser and she said he has gained some weight back in the last couple of days but I am still taking him to the vet nonetheless.
Also yes this vet is an equine nutrition specialist. I did some research on her as well and she is highly recommended by my trainer. She has a list of her specializations on her website and equine dietary and nutritional needs is one of those specializations :]