Keep in mind that filing a lawsuit means there will be a public record at the courthouse revealing some very private details. Unless he has a lot of money, you're likely to spend more suing him than you would get if you won the case.
The "no fees unless we win your case" personal injury attorneys are looking for clear liability, large damages that can be proven, and a defendant who can afford to pay. This is the kind of case where most lawyers would want their fees up front, as would the necessary medical experts.
You might also want to consider just hiring an investigator first - sometimes they can find out if the person had an STD by discreetly interviewing prior partners.
Also, I agree with attorney Marshall about the public records. I've been involved in a few of these cases where the party to be sued didn't want it made public record so they settled early, especially in a small community.
Just be careful and make sure you can prove that he had it before, he knew about it, and that you (as being extra conservative) can prove you didn't have it before.
This is just general information. Not specific advice - I'd have to actually sit down talk and ask questions for specific advice - the above is just how we handled cases like this before.
Good luck with your situation.