I think the BTD theory is that it makes little or no difference to the diet, but I am not so sure myself. I wouldn't be surprised if it was later found that it did. I found this on the BTD site, posting it here in case the link does not copy.
"Hi, ken ~~ Turns out that it plays little or no role in diet & exercise choices. The Rhesus system is a "true" blood typer -- meaning it affects only the blood itself and is not expressed elsewhere. It may be of some small consequence in O's, but they will still follow the O diet.
The ABO system is a separate parameter for typing from the Rhesus system. It's usually expressed throughout the tissues and secretions, so it's the one the Blood Type Diet is based upon."
fsnaturelady - it's interesting that about your son, I think I might try that myself.