Hi Safetymom,
I'm pretty sure the risk is no more than if you donated blood. The process is the same except that the blood is passed through a machine to separate the cells needed from what is not needed. If I remember correctly there may be medication that the donor needs to take for several days leading up to the procedure. I don't know much about this drug, but I think it increases blood cells.
My husband and I donate blood regularly at the Boston's Children's Hospital. One time while we were there, there was someone hooked up to a machine while they were giving blood. I asked the nurse about it and I am pretty sure she called it a peripheral blood donation. I am pretty sure it takes quite a bit longer to do than a regular blood donation. I am pretty sure whatever isn't needed get returned back into the donor through another vein in the other arm.
I don't think there is much risk, but you should do your own research.
Sorry about my awful memory. It was awhile ago and the details are fuzzy.I also remember being told it was a new procedure.
Wishing good luck to your son.