I don’t know what to do..

I need some help. My boyfriend of a year and I had a baby in October 2018 and split up thanksgiving. I didn’t think we would ever get back together so I slept with some guy that was a mutual friend. Well now we are back together getting an apartment. But I have yet to tell him that I slept with someone else during the time we were split up. How do I tell him or when? I don’t want to hold back anymore and I want a clean slate. My guilty conscience can’t take it anymore and I would rather just put it out all on the table now instead of when we get into the new place. Advice and tips please.