Without knowing the particulars, what I can say as a single mother is that it is our responsibility to protect our children as much as we possibly can. Sometimes that includes protecting them from the biological father, biological grandparents, etc.
If the father has already hurt him somehow with thoughtless words, perhaps you can explain to your son his motivation or intentions in a kind and loving way. Try to engender in your son the qualities that his father has never connected with or has lost contact with. I think children have a greater capacity for compassion and this is a perfect opportunity to use, what most likely is confusing and hurtful, as a means to develop a very valuable quality. If this behavior continues, do what you can to distance him from his biological father.
I know it is very difficult as we always want the best for our children and part of that is the ideal family. But the reality is that sometimes the very best family doesn't necessarily include all the biological elements.
I wish you the best and my heart goes out to you.
S.
P.S. I just read the second part. Pursuing child support is a decision that each single parent has to make when faced with a parent who does not want to own responsiblity. It is his choice if he chooses not to be a part of your child's life, unconscious decision though I think it is, but it is his legal obligation to pay child support. As long as he is no threat to you and your son, your son is entitled to the best life he can have and any financial support will ease your financial burden. Do not be discouraged as your son is by far the greatest gift you have been given. He will love and admire you all the more for being the strong woman that you are.