INAL, but I would think as long as your differences overcame your similarities, the burden of proof would be on the person suing. Of course, anyone can sue you for any reason. It's still going to cost you money for attorney and court fees even if it the similarities were a 100% coincidence.
How likely would this person be to sue you? Did you make him a child molester or something equally heinous in the story? Are you telling secrets told in confidence and thinly veiling the circumstances? Are you harming his reputation if the truth were to come out? Is this a revenge fic for you?
If it's completely innocent and you like how this guy looks and something he did and then wove a fictional story around it, I think you'd be less likely to open yourself up to a lawsuit. Again -- I'm not a lawyer.
You may want to consult one before pubbing if you're worried.