So, my munchkiun is in the same boat. He has a BM once a week or less. Have you tried pushing Prunes, Kiwi's and Blueberries?
DS 4 did the same thing when he was potty training. (he was 3 at the time.) He'd go 3 days without pooping and we were spending hours with him sitting on the toilet. I ended up giving him Pedialax-worked like a charm! He couldnt hold it any more. After 2 or 3 weeks he went on his own no problem.