Thor

I presume you were having unprotected sex with him all along? No condoms before?

I think he cheated. You could approach him with your concerns and say that medically it seems impossible that he had it before you met and only passed it to you recently. All the evidence tells you that he was unfaithful. But you want to give him the opportunity to prove the docs wrong, and he can take a polygraph test.

See how he reacts.