I noticed you said the phrase "potty chair". That may be the problem. Unlike tinkle, poop looks and smells yucky. He may prefer using the toilet. This was the case for my daughter who hated seeing it.
Because of my negative experience with Pull Ups and potty chairs with my daughter, I didn't use either with my son. Instead I bought regular undies with his fav characters on them. Told him if he wet or pooped it would make them yucky and he'd have to take them off. Granted, he had accidents but he got the hang of daytime toileting in less than two weeks. Really enjoyed running around the house sans pants showing off his undies.
Also I'm an early childhood major conducting potty teaching research. From my daycare work, I found kids age 2.5-4yrs who use Pull Ups take an average of 5-8 days longer to learn as opposed to the undies group (12 kids each group). This is particilarly true for pooping. Guess maybe because dirtying a Pull Up is similar to soiling a diaper (??). Currently trying to figure out how to understand the children's ideas about that.
Anyways, hope some this info helps. And good luck!