He thinks it's not his...

My boyfriend has always accused me of cheating .. It never fails . Though he knows very well I've never done anything to him behind his back or when we were on and off.. I know what you all are thinking " he's accusing you cus he's probably doing it" and your probably right I've caught him sending stupid shit to girls on a few different occasions .. I let that ride we just haven't been right .. We been officially living together 1 month on the 17th of June without him coming and going . I usually get my period on the 18th and I missed on the 20th I took a test and it came out positive .. YUP I'm pregnant 5 weeks! Now he's trying to do the math and Bla Bla Bla saying how can I be so far along if he just moved in last month..which is ridiculous cus it's not like we were seperated for a month or more he'd come over and we'd still have sex obviously and shit he even has me trying to figure out the time I actually concieved just to shut him up! & make him understand . I'm determining I got pregnant on May 29. What do you all think?