I agree; I think your son has you trained. The behavior he's learned will continue until you put your foot down and tell him no more. In my opinion, you are not expecting enough. As long as there's no developmental issues identified by your pediatrician, you need to take back your position as "parent."
I have a 4.5yo and a 17mo, and they both feed themselves (except for yogurt or something similar for my 17mo), even rice, refried beans, corn, etc. My 17mo isn't real good with a spoon yet, but she uses her fingers or a fork. Both have been doing so since they were less than a year.
Teeth brushing is a little different. My 4.5yo brushes his own teeth, but only because his dentist has said that he's doing a really good job. I check them every once in a while just to make sure, and I still floss for him because he's tried and gets too frustrated. The jury is still out on my 17mo, who we have just recently started brushing with.
Potty training for every kid is different, and is a very touchy subject for most. Boys on average day-train around 38 months, which your son is way beyond. I would do underwear during the day for sure, but I'll let you know what I did with my son. When he was 38 months, I told him that when his diapers run out, he was going to have to wear underwear and use the potty like a big boy. Training up to that point had been sporadic because my daughter was 6wks old (this was at Christmas). President's Day proved to be the day of truth. He knew what it meant when I told him that we were putting his last diaper on, and when we caught him trying to poop in his diaper a little while later, we immediately put him on the potty. He sat that for about 30 minutes, crying and carrying on, but we didn't relent. He finally had no choice but to poop in the potty, and when it splashed his butt, he thought it was hysterical! We've had a couple accidents over the last 14 months, but for the most part, that's all it took.
He still wears a Pull-up at night but he wakes dry 99 times out of 100, and when this bag runs out, we'll be trying underwear at night. If he slept in his own bed, I would've pushed it by now, but he sleeps with us, and we don't have a mattress pad (which I know I should just get one) and a really good mattress. In fact, the only 2 accidents he's ever had at night have been in that bed!!!
I'm a big proponent of doing what's right for your family, but in this instance, I think you should be expecting more of him. You don't have to be mean about it, but I think you should be firm in your beliefs and actions, otherwise, he'll see the slightly opening and know that you won't follow through.