My son was potty trained at 3. Brutal, long, demanding process. The only thing that worked with both my daughter (trained at 2.5) and my son was to get rid of the pull-ups. I cannot stress that enough. They are great as an intro to potty training, but with what you are describing it sounds to me like it is time to ditch 'em. Yeah, he may have an accident in them but underwear is much different than pull-ups that he will stop going in them.
Think about it, he has been peeing & pooping in a diaper his whole entire life. A pull-up is still a diaper and he is conditioned to go potty in it. Underwear is totally new to him, so he will slowly begin to understand that these different things need to be treated differently - at night too. He'll get it.
One of the developmental readiness signs of potty training is if the diaper is dry in the morning. If his diaper has not been dry, they he may not be physically ready yet. If dry in the morning, do not give him a diaper, aka pull-up, at night either! If he is physically ready, he can do it. Now mind you there are some children who cannot stay dry at night for medical reasons.
Good luck.
A.