I'd continue to ask around, any veterinary schools nearby that might make a home visit? From what I've read it doesn't seem like tumeric would hurt, but it probably wouldn't help either - they are still studying it, trials are underway.

What stresses him out about vet visits? Is it the cat carrier? The car ride? Maybe you can work on getting him to see the carrier as okay, then take him on short car rides to let him know it's not scary ...

But as it's an external tumor, on his paw, surgery might be needed, but they might be able to do local anesthesia - you really would need to see a vet.