Here's how the state department defines the evidence:
(2) A bona fide parent-child relationship exists or has existed while the
child was under the age of 21.
(Such a relationship exists or has existed when the father displays clearly
or has displayed clearly an active concern for the child's support,
instruction, and welfare. Documents to manifest this concern may
include (but are not limited to) the child's birth certificate, local civil
records, affidavits from knowledgeable persons, and evidence of financial
support. DHS may require blood tests from the petitioner, beneficiary,
and the beneficiary's mother.)
http://www.state.gov/documents/organization/88018.pdf
If the father never supported the child and wasn't in contact with him all these years it may be impossible to prove a bona vide relationship. From what the OP mentioned, he only recently reconnected and didn't have a relationship with his son all these years.