Of course this would upset me. I'm not sure how it couldn't upset a person. It would be a shock at the very least and any normal person would start thinking, how will this affect our lives and family? But separate the emotions from the issue.
The woman (and who she slept with) is her business. I wouldn't even go there. The other possible fathers really is irrelevant. I don't get the whole 'go have them all tested' thing (I get you didn't have the funds for the testing - however, to me that's irrelevant.)
So what would I do if I were in your husband's shoes? I would just be tested. What else is there to do? I would a) want to know and b) it's the right thing to do.
As for what do you do after that? If it turned out I was the father? I would personally consult a lawyer. I would want to know if I was financially responsible for the boy.
Obviously - I'd likely want to meet the boy and go from there. It really would depend on how much involvement I (your husband) would want and the boy would want. Then from there, how involved everyone would want to be (if you and your family would want to meet him, etc.).
The whole emotional side of this is very complex .. and I'm not sure anyone can relate to this unless they've been through it. I have friends who have met their biological mothers, etc. (who were adopted). It is very involved, and very emotional. Typically, they just want to meet so that that they can fill an emotional need (? not sure if that's the right word) or curiosity (? again not sure if that's the right word). Without having experienced it, I doubt any of us can know what it feels like.
I would just be supportive of your husband during this process, as I'm sure you are.
Being angry at the woman I think is pointless (but understandable). Just let that go - feel it, and then let it out - let it wash over you and release it - don't carry that around with you. Consider she was young (I'm guessing) and clueless at the time (possibly scared, who knows) and she made the best of the situation at the time. Perhaps she wants the best for her child like any mother would now - and that's to give him an answer she feels he deserves. She is doing the best she can - which is all we try to do. It is kind of messed up - but sometimes life is.