Would you stay or would you go?

My boyfriend has a baby on the way only thing is it's with his ex we were dating for two years I moved away and left him Bc I didn't feel it was fair for him not to see me win I came back he left her n started datin me again,we found out she wa pregnant but also told him it might not be his.If this is his idk what to do I dnt know I I can look intothis Childs eyes knowing it's not mine,should I stay or go.I do love him n am in love with him but it is already dragging me down mentally because idk what's going to happen if it's his ,Shea two months till her due date .