Some people will still retain their fertility after chemotherapy and radiation therapy, so it is not altogether impossible that your son has fathered a child. Unfortunately, the only way to be certain regarding the issue of paternity is for both he and the baby to be tested.
As for the impact of your son's diagnosis and treatment for AML, there may be some increased risk of genetic abnormalities in offspring following parental chemotherapy but, once again, it is not possible for me to say what the risks are specifically for this new baby.
Sincerely/ Robert A. Wascher, MD, FACS