Will a baby make him run?

Me and my boyfriend broke up in December, but we still stayed close and talked everyday. Well during that time we were broken up I slept with another guy to try and get over my boyfriend and the condom ended up breaking on my fertile day, I wasn't really worried about it because the guy said he hadn't finished in the condom anyways and not to worry about taking plan b... Anyways I stopped talking to the new guy and me and my ex decided to get back together, but now I'm worried that I'm pregnant (my period is due today but I haven't had any symptoms or signs it's going to start). And if I am pregnant there's a big chance that baby might be the guy that I slept with when me and my boyfriend were broken up. But also a chance it could be my boyfriends... although me and my boyfriend have been using the withdrawal method for over a year and a half my periods were so irregular that I wasn't ovulating regularly, until about 2 months ago. I told my boyfriend that while we were broken up that I had sex with someone else but not that the condom broke... I've never cheated on him but I feel like if I did end up pregnant without knowing who the father was he would just straight up leave me... I just need some advice should I wait for my period to be a couple days late to tell him or tell him now that there's a chance I could be pregnant from the other guy... I don't want him to leave but I don't want to lie to him either... And if I am pregnant how soon wouldd I be able to tell who the father is?