Sorry to hear about your boys diagnosis.
My girl dealt with over 16 mast cell tumors in her life and over the course of 4 years.
We never utilized this type of treatment as it wasn't approved at the time and all of her tumors were able to be removed via surgery or killed off by Vetalog injections (far less invasive than surgery and much less expensive as well). Once we found out about the injections we stuck to that treatment with 100% success and no additional surgeries.
Kinavet - CA1 is most commonly used to possibly treat tumors where surgery isn't an option. This can be caused by location of the tumor or the patients health (surgery would be too risky). These results are from recurrent tumors (surgically removed but returned later), aggressive grade 2's and 3's.
I believe it offers a 50% success rate in shrinking tumors but not sure what percentage it offers in completely killing off the tumors.
Hopefully you are using the expertise and guidance of an oncologist and not your traditional vet??
Most common side effects will be nausea, vomiting, diarrhea so be especially watchful and even proactive to ward off these symptoms if you can.