I had the same problem when I brought home my youngest. I had Tiger on G-man's 3rd birthday. My son was completely trained until my mother watched him and left him in pull ups the two days I was in the hospital then oh boy. It's really an attention getter at this point.
The best thing to do is to stay as emotionless as possible. Just clean him up, set him on the potty for a few minutes and go on. No negative comments or mad faces--Poker Face Mom. After all he like negative attetion as well as positive attheniotn. Is it possible to let him go bottomless right now? Do it. Or just put him in pants and leave him even if he's wet.
Now since you're retraining him, set a timer for every 15 minutes instead of waiting every 30 and don't ask, just take him. Infact, stop asking questions all together because constant questions create a feeling of insecurity in a child. It's going to require extra effort but you'll get faster results.
For me I "wore" my youngest unless I was going to potty. That made it easier to just take the middle boy to the potty. I didn't have to worry about upsetting the baby.