Dear Lili,
Hi. Sorry to hear of your troubles. As Clark noted above, your son's doctor should be able to give you clearer guidance on this. I'm not a medical professional, but from what I have read the problem is not so much the chemo treatments but rather the possibility that some remaining active cancer cells might be passed along with the transplanted kidney. For that reason the doctors sometimes insist that donors be entirely cancer-free for some years before trying to donate. It might be wise to start getting the word out now to search for alternate donors if it turns out your ability to donate is postponed.
Keep in mind also that successful kidney transplants don't usually last forever. Many recipients may need a second transplant years down the road (10? 15? 20?), and some successful transplants fail earlier. So even if your ability to donate were delayed for a few years to make quite sure you are cancer-free, you might still be needed to donate to him at a later date.
best wishes,
Fr. Pat