Potty training help

We tried the three day potty training method for our son about a month and a half ago, it seem to be going OK. He was not have any wet accidents during the day. But he won’t poop on the potty at all. Since then he has gone poop a few times on the potty but is holding it until he has a diaper on again at night or until the last possible minute and it’s torture. Within the last two weeks he has started to ask for diapers again and started peeing in his pull-up. I am at a loss, we are due with her second in two months and I really wanted him potty trained before then. Should I just take a step back and wait a little bit longer?