You might find this previous MedHelp post and followup useful:
Arnold L. Goldman DVM, MS
MedHelp & PDOC
In most cases Mast Cell tumors once removed are fine. They may come back, but if they do, it is locally as they do not metastasize or spread through the body.
Although each veterinarian has his or her own protocols as far as therapy for mast Cell Tumors, in my experience, once removed, most dogs, 99%, plus are fine.
Because the margins are clean, your vet did a good job and chances of recurrence are possible but not highly probable.
We recommend altering diet to either home-made or organic and boost our canine patients internal immunity to help prevent recurrences. We has had very good results using with an all-natural vitamin supplement, called Paaws, which is also patented for dogs.
We have not had to resort to chemo or radiation for our canine patients with this diagnosis.
Because your dog is young, consideration of natural, holistic therapies would be well warranted in my opinion and help him to enjoy many more happy, healthy years with you.
I hope this information is somewhat helpful and would be glad to discuss this with you further.
Dr. Carol Osborne, DVM