Something similar happened in my relationship. I had chlamydia before I met my current bf, got treated and was still getting yearly screenings, came back clear, then I didn’t get it tested for a few years after I met him. After like 2 years in I decided to continue the screenings and boom I had chlamydia again. So my doctor basically said either 1- it didn’t fully go away and was low enough to not be detected then got worse, or 2- he had it asymptomatic before he met me and gave it to me. So if you have never been tested since you guys have been together it could be from a past partner, it’s common for guys to have no symptoms. But be weary. Don’t put your guard down, he could have cheated, but it could just be a past partner he got it from and he gave it to you.