Only one has swatted back? I'd not hold him back and let them fight it out themselves. They are bound to all swat back and give him a good swipe on the nose. All of mine would have done so in the first ten minutes of him being in the house! Seriously, I've found that the animals always work things out among themselves. Unless your cats are especially scaredy cats or feral, the "let them be" theory works best.

Good luck at the vet with the lump.