I teach Abstinence education, and I have never read in any of my materials anything that says you may contract chlamydia in any way but sex. I don't think that diagnisis could be a mistake, but I'm not a doctor either. She could always get another opinion. Please don't be offended, but it may not be your son who has had more than one sexual partner. If either one has been sexually active with even one other person, they may have contracted it.