Infidelity Support Group
Any relationship in which one partner engages willfully in sexual relations with another outside of the partnership is considered to have experienced infidelity. This breach of trust is often traumatizing for the faithful partner as well as the relationship, and support is often needed to heal emotionally and to decide whether or not the relationship should continue after...
I can't say for sure what I'd do if I was 19 and pregnant by a complete stranger, but I would think he would have heard from her already if she had gotten pregnant and decided to keep it -- I would think she'd want child support unless she's wealthy enough to raise a child by herself, which I tend to doubt.
All in all I think the inconsistencies of his story is what's bothering you, perhaps more so than the angst of wondering whether he fathered a child.
Bottom line, I think, is you two have some more talking to do.