I'm so glad this post got bumped! We're having the same issue with 19mo DS - he had a traumatic poo (was given a brownie and then didn't poop for 7 days despite our efforts to get him moving (we are GF and both times he's had gluten it has stopped him up horribly), then when he did go it was painful) and has been withholding since. It's so stressful and maddening to see him walk around on his toes, clench, do anything in his power to keep it in. Flax oil does help, but he still mostly nurses and doesn't eat a ton of solid food, so it's almost impossible to get him to take 2 tsp a day.
Off to research!