Why do you believe your illness would preclude you from continuing to enjoy time with your son? There are plenty of people with various medical ailments and chronic health problems who are still able to provide loving caring homes with good parenting.
I believe you are afraid that if your cancer reappears, you would not be able to care for your child and, as a consequence, you would lose physical custody. Unfortunately, if your child is young and you would be unable to care for it, this could happen. But if you would live with someone who could help you, this would probably be considered by the court. I hope you are cancer free for good.