he won't be at pull-ups at his 5th birthday party.
I guess that he's just not ready to learn yet. Some children learn later and others earlier. I have the oposite scenario. My firstborn didn't learn until he was over 3 but number 2 is learning already and he's only just 2.
Putting pressure on when he's not ready will just stress you both out so maybe the best thing to do is just put him back in nappies and wait for while then try again.
They can often regress with change so if you do teach him in a few months he might go backwards again after your baby arrives.