Best time for boyfriend to not use condom

I am on the pill and I take it at the same time every day. My bf really wants to come in me at least once. I know that theoretically the pill makes you not ovulate so anytime would be a good time to do that. However I have IBS with frequent diarrhea so I think the pill is less effective in me. So what time in my cycle would be the least likely for pregnancy to allow him to do this?