Please someone shed some light

I’m struggling so much at the moment! I’m currently 19 weeks and my partner just can’t deal with the pregnancy, he thinks the baby’s not his because I had unprotected sex three weeks before I had sex with him, my period was in between the 3 weeks, at my ultrasounds they’ve already said the baby’s a little bit smaller then normal but wouldn’t that mean I’m not as far along as they’re dating me? I want the baby to be my partners and I know he will be around either way but he doesn’t want to bond with the baby and get his hopes up if the baby turns out not to be his. Is there any possibility it could be the other guys or is the baby definitely my partners considering my period came after sleeping with the other guy and also what they have dated me? I track my periods so I knew exactly all the dates. I know this sounds gross but my partner and I were also sleeping together while I was on my period. Can someone please shed some light on my horrible situation because I’m sick of feeling so guilty and I’m sick of my partner feeling so down and out. We will be doing a DNA test I just want to give my partner reinsurance.