Q. What are potential risks when removing a mast cell tumor in a feline
A. With any surgery there are risks and complications that can occur; anesthetic, bleeding, infection etc. But having the surgery performed by a skillful vet these complications are pretty minor when compared to the benefits. Mass cell tumors have the best prognosis if they can be successfully removed. It is often an aggressive cancer so getting rid of it is usually the best option for treatment (sometimes combined with other treatment).
Your cat is not very old so it is likely a fairly good surgery candidate and has the potential for a quality life afterwards. If surgery is a viable option for him (and you can afford it) and that is what your vet feels is required I would try it