Your pediatrician is right. And I am not so sure they will get really concerned until he is closer to 5 years old. This is your son's project and not yours. And as much as you want him poopy trained now, you just need to relax and perhaps lay off a bit--as frustrating as it is .
I know too watching other kids his age fully trained is frustrating but your son is more mormal than you think. However, if you are noticing other signs like he personally is very upset about what is going on (not because you are) then I would address this with his dr. Hang in there
. My son just made the decision to be potty trained after going back and forth. I've been told that it is easier for most kids to potty train than poopy train. Again, relax, this is his project and he still has some time.