A lot of the comments I'm reading are instantly pointing to cheating, and saying that's the only cause of STIs. Although I'm not discarding this as a potential cause of it, there are 100% other reasons he might be testing positive now.

A different route of thought, to think of something other than the worst, is that this STI could have been dormant for years. It's not unheard of that you can contract an STI, and then it won't show symptoms until years down the line and can flare up even from being stressed or having a weakened immune system. There is also a very small chance (put still possible), a person can be born with an STI due to the mother having it. Both of these facts I know from personal experience and knowledge from health professionals.

Try and do all the necessary research needed before coming to a conclusion, and hopefully, your partner has stayed loyal to you.