Pooing

My son just turned 4 months. For the last 2 and a half months his pooing is becoming a lot less frequent. It was every time a I changed his Nappy, which was after every sleep, then he began pooing every 4 to 5 days and most recently he hadn't pooped for 15 days. And currently since his last poo it has been 10 days. I did start feeding him at 3 months as he just wanted food, he eats really well but I had to stop giving him food and by food I mean pureed food in a pouch. He is exclusively breast fed. I've taken him to the doctors but they said because he is still peeing, eating and generally happy that he is fine. Is there any Info or anything I can give him to help him poo more often.