M., we JUST went through this a few months ago...potty training is SUCH a big step, i hope you can figure out what works for him!
i would absolutely not use pull ups except at night or during naptimes. to me it's a waste - they are SO much more expensive than diapers and not much more convenient. towards the end i was putting on and taking off my son's diapers without him even laying down.
i did try putting him in undies and hounding him, but he was literally having accidents every 20 or 30 minutes, i don't know what the deal was, maybe he wasn't finishing on the potty. anyway, one day it just clicked for him. he started telling us and it was like magic, how quickly things turned around.
the last time when it finally clicked, i had been leaving him in diapers and just trying to ask him, every hour or so. i would praise like crazy when he would be dry, and be "sad" when he was wet. not angry or judging...i'd just look sad and say something like, "oooh. not dry. that's okay, we'll try again..." when he came to me and told me he wanted to go, was when i started putting him in undies. then i made it very clear that (whoever was on the undies) would get soooo sad if they got pee-pee on them...and it seemed like overnight (i think it was more like a week or two) he did a complete 180.
i don't really have any advice, just use your common sense...and you'll know when he's ready. but keep trying till then...you never know when it'll sink in!! GOOD LUCK!! i am so excited for you, it's such a relief to get past this hurdle...!