I am sorry about your cat!
Many dermal mast cell tumors in cats are benign and surgical excision may even be curative. Surgical excision can often be performed with a local anesthetic if the mass is not too large, with or without a mild sedation. This type of surgery is fast, not overly invasive and, is not very costly.
If a splenic mast cell tumor is suspected, X-Rays and then an abdominal ultrasound should be performed. Surgical removal of a splenic mass would be an invasive surgery.
If it is positive that your cat has a splenic or other abdominal mass and you do not want to have him undergo surgery, than there are holistic anti-cancer treatments than can be used. The best option would be to seek assistance from a holistic vet, or if your present vet does holistic treatments you can inquire from him/her about the use of Neoplasene oral anti-cancer therapy. Other anti-cancer options are: Poly MVA available online from polymva.com, Hoxsey Formula (available online), adding Turmeric to your cats diet and placing him on a cancer diet (which is also diabetic friendly) among other options, such as Chinese herbal anti-cancer remedies. All of the listed options are diabetic friendly.
If you are interested in the anti-cancer diet and herbal remedies please post again.
For information about veterinary help and schools in your area please post to the medhelp vet community.