Has the vet biopsied the tumor? It may very well be just a benign polyp or growth that they can snip off with an endoscope.
Before making any decisions I'd for sure get it biopsied.
If it does turn out to be malignant (which I pray it isn't!) then maybe talk to them about sending the horse to a vet school or university so that they can help him.
Spent a whole hour today laying in a pasture, waiting for a sparkling vampire to show up. Alas, I woke up and looked over, only to find a mound of horse crap. Sigh.