T.T.
I dont have this exact prob. since we havent started Potty training yet.
But I do agree that it is a learned behavor.
The prob is not really constipation is that of he is holding it until he cant hold it anymore. He isnt ready to go poopy in the potty and is holding it.
Before I would go through the tramatic experience of a supository I would go with Miralax.
My DD has been on it since she was a year old and has been on it everyday since for a year now. She has rabbit like poo and has constipation issues herself just like mom.
I would go find some potty training books on this issue. I have read about this I think in the "everything you need to know about potty training" book I found it at Barns and Noble.
Good luck.