I just wanted to add a message of hope and encouragement - I didn't want you all despairing after reading Alibubbles' message that her son still has problems. It is a dreadful situation to be in, and hard to get support. Hang on in there ...
My eldest was always prone to constipation, from 5 months when he started on solids. It was always a battle, and a constant watch on what foods he ate. He never got so chronically bunged up as some of the children below, but it still got to the point that it dominated our lives. When he was constipated he was in pain, and his behaviour deteriorated rapidly too. I was always aware of when he had last done a poo, and how critical the situation was. Whether or not he'd done a poo would dictate how we spent the day ...!
Anyway - to the 'hope' bit. There is no problem for us at all now. He is nearly 4.5. I do make sure he has plenty of fibre in his diet, but don't worry further than that. I no longer necessarily know when he does a poo, and I don't constantly think about it - it is simply not an issue.
How we got there was by making a massive amount of fibre in his diet (recipe to follow), so that he simply could not hold on. And then insisting - with rewards - that he sat on the toilet until he did a poo every morning. At this time he got to listen to cassettes with stories on - the only time of day we did this. Once he was regular, it stopped causing him pain, and we just gradually scaled back the pressure until he went by himself.
I do think a lot of the problem is psychological - sometimes they pick up negative vibes from you, about mess and so on.
Two other stories of 'hope'. My younger son has never had a problem with constipation. He's now 2.5, and out of nappies, clean and dry. He takes himself off to do a poo twice a day. So don't worry that you will necessarily go through all this again with another child!
Another friend of my son got chronically constipated, and nothing seemed to help. His mum was convinced it was psychological, and certainly no dietary changes, or medication seemed to make a lot of difference. Then one day when he was 4 he saw his young cousin (aged 2) do a poo on the potty. Since that day he has not been constipated at all, and takes himself off to do a poo when he needs to without prompting! Honest - it's an amazing story isn't it? But sometimes it just takes time and a change of attitude of the child.
Good luck all of you, let us know how you get on