Your input is very valuable. Melanomas in horses are different than melanomas in people. You will read more about that in the case studies on this site, and in the links to other case studies. This is because each time the tumor splits, it doubles itself. If your vet is skeptical of Cimetidine treatments, or isn't familiar with it, or if he drags his feet about getting information - call another vet.
Vets are human.
Most of them work 18 hours a day already. My original vet has come to my horses' aid on many occasions, and he is one of the most highly respected vets in our area. But it is impossible for ANY vet to have all the answers. If I had moved more quickly, there is a good possibility that I would have Garbosa around for a long time to come. Oh, and one parting thought on Andalusians and melanomas.