I have Multiple Myeloma (similar rare cancer) as part of my treatment I was taking Famvir to both prevent herpes outbreaks, and also shingles. He may be already on an anti viral, it's part of the cocktail of meds given in support of chemo. I had several outbreaks even being on this med (mostly cold sores for me) but the doctors were not that concerned, they just upped the dose to the treatment dose (two pills instead of one for a few days)
That would not be a good thing if he contracted Herpes from you because his immune system is compromised already.