Hi B., I know you probably don't won't to hear this, but the best thing you can do is to stop buying pullups and diapers immediately. I have 5 kids, ages 24, 13, 11, 8 and 8 and 3 grandkids ages 4, 18 months, and 4 months, and I was also a foster parent for 8 years, so I feel like I am quite experienced in this matter. LOL I went through this with my 24 year old and my 8 year old and just went through it with the 4 year old grand daughter. It seems like when they have the security of the diaper or pullup (same thing actually) they feel like they have "permission" to use it in the diaper. If you will put underwear on him as soon as he gets up in the morning and make him continue to wear them all day even if you have to change his clothes and underpants 13 times each day, it will only take a few days to a week before he figure out that peeing and pooping in underwear is not pleasant. You can use pull ups or diapers at night, but for consistency, I would recommend that you buy some rubber pants (available in toddler sizes at walmart in neat patterns) and put them on over the undies at night and place an adult underpad on the bed under him. My grand daughter turned 4 in October and was still using pull ups. Her mom tried everything in the book just like you did and when I finally convinced her to make her wear underwear she started going to the potty after about the 2nd day. Now my daughter says she wishes she had listened to me 2 years ago when I told her to throw the pull ups out. She still has occasional accidents at night, which is to be expected but we just change her panties and change the pad without making a big fuss about it because we know that night dryness will come as she gets a little older. A lot of 4 year old have night time accidents so don't be surprised if you have to deal with this for a while longer. Also, the best reward for him using the potty like a big boy is for you to praise him and tell him how proud you are of him. Kids this age love to get the approval and praise of their parents and try really hard to get it.
Hope this helps some.
V.