SEN 5yo DS with toilet issues

[Crispspsps]

Hi - my DS only got out of pull-ups at Easter - it was a major victory for us as his toilet training was one of the main things on his SEN profile (He also has some - relatively mild - social and emotional flags ticked: he gets anxious with new adults, his speech is behind his peers, but generally since starting school he has become more confident all round)

He was weeing on the loo really well, and a few days in did a massive poo. I mean MASSIVE. He has always been irregular - a week with no poo then loads over a few days. But now, although he will ask to go for a wee, he is holding back the poo. Took him to the GP on Friday and she said he is very impacted, so we have upped the Movicol, and it's dripping out of him all the time. I resorted to putting him back in pull ups this afternoon as I had to throw out the pants, they were so soiled.
He just will not poo on the toilet. I am feeling like such a bad mum - his SEN issues mean he can understand that he needs to poo on the toilet, but when faced with the reality it's all "NO!"
I've tried talking, bribing, even physically putting him on the loo (which I know is awful and I hate myself) and ignoring it (but he's too old to wander around with no pants - he demands clothes). And today I totally broke down. He came and gave me a massive hug, the sweetheart, and said "If I do a poo on the toilet you won't cry". Broke my heart. I don't want to be emotionally blackmailing my gorgeous loving 5yo.

I just don't know what to do next. He's supposed to be at school tomorrow, but the constipation/Movicol is making the poo drip out constantly and I can't ask the teachers to change him every 20 mins. I also don't want to send him to school in pull ups if I can avoid it, as he was so proud to go to school in pants and just got the hang of going to the loo at school.

Any ideas? I'm a bit at my wits end. x

[Squeezed]

If the constipation /impaction has cleared then you should be able to reduce the dose so that it isn't so loose. Obviously speak to gp about it. The problem from impaction is that they can lose the sensation of having a bowel movement so unfortunately he may have less control than he did. This may take some time to get better.
If he's worried about going to the toilet then give ERIC website a try and see of the Poo goes to Pooland ap, dd loves it.