First of all, I'd be p*ssed that he was sleeping with other people.
Second, I'd be p*ssed at him and myself for not using a condom.
Then, I'd be glad it is something curable...cuz there's other things that are worse, like syphillis and herpes that can affect the reproductive system...
How to tell him?
If you guys are having sex, which is the physically closest thing you can do with someone, you should be able to say..."hey, WE have a problem. I had a doctor appointment and confirmed that I have chlamydia, and your the only one I've been with. I contracted it from you.
This is really uncool...and now, you need to tell whoever else you are having sex with, that they should get tested and get some meds, or they will be giving it to others.
Oh, and you should get some condoms...cuz even if you take meds to clear the chlamydia up...your Boyfriend still has it, even if it's dormant and he can give it to you again...
I'm not a prude...believe me, I love sex like most people, but the bareback version (no condom) isn't worth getting diseases which may have long term consequences...some of those STDs are really nasty. Google: herpes, syphillis, gonorrhea. All will do major damage. Herpes will be with you FOREVER...
Good luck to ya.