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Thanks for your question.
The only definitive way to determine whether your son is the father of this child is through genetic testing. You can use a competent lab to perform the genetic testing and obtain a result for your own peace of mind, but, to legally establish the child's paternity, your son, the child's mother, or the Department of Social Services if the mother is receiving public assistance would have to file a paternity action in Family Court.
If your son has any doubts whatsoever that he is child's father, I would strongly recommend genetic testing. And, by filing a paternity action and having a judge order genetic testing, if the results come back with at least a 95% probability that your son is the father, the judge will enter an order that declares him to be the child's legal father.
Hope this helps. Good luck.