That may be the reason for the lower Hb. There may be other genetic factors keeping the donor Hb higher, or it may just be the younger age, but the son who had the BMT will likely have the characteristics of a minor. I've seen that before with a friend from the Netherlands, who had a BMT with her minor sister as the donor in the early 90's, and her Hb was that of a typical thal minor after the BMT.