Need advice about husband's son

[onlyadream]

I'm so sickened that I don't know what to do.
Here's the lowdown:
My husband and I have been together since 1990, now married 17 years with two kids (15 and 12). After we were together for 6 months, he broke down and told me that his ex had a baby and claimed he was the dad. They broke up due to her infidelity, so he was convinced that the baby wasn't his. I asked a lot of questions, feeling somewhat betrayed having been kept in the dark for those months. The baby was born about a month after we started dating. I don't remember what happened after that, but it has been in the back of my mind for many years, however, I really thought (or WANTED to think) that he was right, that he wasn't the father. In 1999, we received papers; the step-dad wanted to adopt. My husband signed and that was the end of it.
Well, the other day the universe spoke and literally put information in front of my face, which (with a little sleuthing) led me to find the now 21 year old boy on the internet. He is the spitting image of my husband, and what's more is that he is in college in the SAME two majors (double major) that my husband was in when I met him. There is NO DOUBT now that this is my husband's child.
I'm sickened. My thoughts about my husband are horrific. How could he? How could he!!!??? With more sleuthing, I find that the mom was ONLY 17. I'm even sicker now. HOW COULD HE!!!??? I told him that he DID have a son and his reaction was visceral. He calmed down and slowly, each day, he's asking questions (but still not wanting to view the pictures I've found - I think it would be too much). I want to know more - did he EVER give so much as a dime to this boy? - but it's not my place (or is it?) I don't understand why they didn't have a paternity test - or whatever. I can't believe my husband is a huge deadbeat. The boy is in his 3rd year of college, and I honestly would love to send a check to pay for the rest of his schooling, but (again) that's not my place (or is it?). He looks to be a great boy, REALLY great. I feel sorry that my husband missed out on knowing him.
So, what should I do (if anything)? There's also the entire issue of how this relates to my kids.