I can see your delimna, but I would think you have to try something. The DMSO is a super penetrant and anything you add it do will very quickly penetrate into the tumor. Maybe try it with different things - iodine, baking soda, black salve?? Sorry to say, but it is really guesswork here, as surely you must know.
Whatever DMSO he gets down will also help with absorpiton of whatever else you are giving him. Insofar as the amount of colloidal silver, I don't think you can give too much up to the limit of what he will drink/tolerate. I have mixed it 50/50 with dogs drinking water before for many days in a row after a rabid skunk attack.