Wow... he potties 20 times a day? Crazy. Be that as it may, my daughter is nearly 3 years, 9 months and we just tossed the potty chair a few days ago. She would have likely been willing to ditch it earlier, but I didn't mind and wasn't going to rush until she felt very comfortable sitting on the regular toilet. That said, she was never averse to sitting on public toilets or toilets at other people's homes, etc. We used every kind of bathroom on the planet as part of "potty training" (precisely because we didn't want to get stuck with her only being willing to potty in a particular potty chair.) We never had one of those seat adapters.. because we happen to have an ADA toilet in our bathroom that is much higher off the ground that the average toilet, it's a pretty big climb for my girl to get up there and sit... only recently, with a short stool, has she really been able to do that comfortably.
Since she was three, I've been encouraging her to poop on the toilet (at home she's preferred to potty on the potty chair.) Just last week I said, "wow, if we got a little step stool, you would probably use this all the time... you want to try that?" She said, "yes" and we were off. When she was about three, I removed the potty chair once and put in a stool.. and she just wasn't really liking it.. so I pulled the potty chair back out again and just gave it time.
So from my perspective, it's less an issue that you rush him giving up the potty chair and home and MORE of an issue that you get him sitting on and using toilets, everywhere. Go back to simple rewards.. if he's willing to sit on or try to use the toilet in the gas station, grocery store, playground, etc. he gets a sticker.. even if he doesn't have to potty, etc. As he realizes it's safe and no big deal AND he's rewarded for it.. eventually it will just become part of "normal." Every day when you drop him off at daycare, have him go use the toilet with you and reward him just for sitting on it. I bet in time, he'll be willing to use it. Praise him mercilessly when he does ;)