It might be wise to talk more with his doctor and get a better idea of the chances of success of the transplant, and of what it might reasonably be expected to do to benefit him. Each patient is different, and some have better expectations than others. It's a big decision, and you need all the info to make it well.
Most (all?) transplant hospitals understand very well if an intended donor, after thinking it over more, decides not to continue. The health testing demands for a donor are so strict that it is quite possible for the doctor to take another look at all your test numbers and decide that they are not QUITE good enough to approve you after all. So nobody else needs to know if it is your decision to not continue.
It may also be possible to look for an older donor (e.g. 50's or 60's) among his friends, neighbors, church/synagogue members, old alumni, former co-workers, etc. Sometimes there are people who are now pretty free of future family responsibilities who would gladly donate IF they KNEW of his need and of the possibility of living donation. Don't be shy about getting the word out. You are definitely NOT the ONLY possible living donor.
best wishes,
Fr. Pat