How Is Urethritis Infection Transmitted Usually?

I have been dating my boyfriend for a year and half as of yesterday, October 9. He is the only person I have ever done anything sexual with, and he claims I am the only one he s done anything with as well. Last week, he told me his mother had chlamydia, and that he has been eating and drinking after her a lot lately. He said that she got very worried because of this, and scheduled him a doctors appointment, and he was very paranoid thinking he would have chlamydia now since she does and he ate after her. When he went to the doctor though, he was diagnosed with urethritis. I googled it and everything I looked at said it was a sexually transmitted infection, but he is completely denying ever doing anything sexual with anyone except me and says that he got it from eating and drinking after his mom. Is this a possible way to get urethritis, or is he lying to me?