V.B.
Just be patient and watch him for signs. Did he tend to hide when he pooped in a diaper? Was there a particular place he liked to go at that time? Watch for him to do those same behaviors and rush him to the potty so that you can catch him and be excited to put it where it needs to go.
Even if he goes in underpants, take him with you to the bathroom (no scolding or anger) and show him how it gets dumped into the potty. If you catch him in the middle, put him on the potty anyway.
Stay positive. Be patient. And no, he's not too young.