Don't know if this post will help at all...
Both ds2 and dd had a problem with pooing following painful experiences with anal fissures. Dd was particularly bad: she was chronically constipated for 18 months and every time she had to poo, it was agony for her. I well remember watching her, knowing a poo was coming (for days!) and watching her resisting it and getting more and more uncomfortable and distressed. I used to plead with her to do a poo. She was on lactulose, sennakot and glycerin suppositories.
Apparently dh had a similar problem with constipation when he was a child and I remember my MIL telling me she was told the same as you Skinnycow, basically that the probelm was hers because she was so worked up by her son's constipation and toilet habits. She was as outraged as you at this suggestion. But then she relaxed about it and the problem did resolve itself.
When she told me this, I did my best to stop asking dd if she needed a poo and pleading with her to poo. It was hard for me, bt I jsut ignored her when she lay on the ground in an attempt to hold the poo in. Also, I didn't get distressed when she screamed when the poo came out. I just tried to be normal and say "Oh good, you have done a poo" to teach her that pooing was a normal thing and nothing special. Eventually her problem went away too, although hers was mainly about the pain she associated with pooing and eventually, I could tell, the pain was lessening for her.
What I am trying to say is that both my MIL and myself did find that relaxing about the problem and pretending nothing was wrong, did seem to help the process. I hope you won't think I am telling you you are over-reacting because your sn's problem does sound worse than what my MIL and I had to deal with, but it just strucj me as intersting that your psychologist and my MIL's doctor both said the same thing.
BTW, I had to have picolax once before an operation. I had a double does and was on the loo continuously going for half an hour when it first kicked in and then on and off all night. Horrible, horrible stuff. Your son is so brave to endure that!