J.N.
My son wasn't potty trained until several months past 4, and a lot of it was becuase of the on-and-off training and it became a power struggle. We just finally put away the undies and went back to diapers. And took away a couple of "big boy" activities he kinda liked, because big boys go potty. Then we didn't mention potty at all, except if he wanted big boy activities, and we just said "Oh, that's for big boys. Big boys go potty." And left it at that. When we asked him, a few months after he was 4, if he wanted to try pottying again, he was ready. And he was 100% trained in less than a week.
When is baby due? He will revert about 6 months back when baby comes. And when daddy comes home, he will also revert some (even though it's a good thing). Just be ready for that. It may be easier in the long run to wait until after baby is born.