Soiling in a 6 yr old - at wit's end - please help

[HopingThisIsJustAStageBut]

DS has been soiling for 3 years, since potty training. He has never really got the hang of it, but is worse now than he was 18 months ago.

He soils daily at school at the moment. He was slightly impacted and is on Movicol at the moment (only 1 sachet a day). I think when he was younger he was so loose that he couldn't control them and this has led to trying to withhold and impaction. Sometimes I think it is a physical problem and other times I think it is emotional. Sometimes he feels he needs to go - other times he claims not to (but is fibbing a bit at the moment so I am not sure which to believe really). Today we had 8 accidents . We have tried charts (sometimes the stress of trying to achieve the treat at the end makes him worse) etc. He goes on the loo after each meal with a balloon (which is meant to help).

He had started potty training well and then a particularly bad accident set us off on this horrible trail.

There is so much more I could write here, but I don't want to bore you to death. Has anyone any success stories - please? When I have googled this I keep reading stories of boys of 16 still with this problem and it is thoroughly depressing.

My son is 31/2 and never really got truly potty trained - I always had to take him to the loo regularly. Then the soiling started about a year ago...we were getting frustrated with him and thinking it was "behavourial" when I came across a website about school toilets, and how important they are etc, because if children aren't allowed to go when they need to, problems like chronic constipation can start - which was when I twigged what DS2's problem was. He saw a specialist privatley and she was great, explaining how chronic constipation that lead to diarrhoea (sp?) and all his symptoms fitted - lethargy, lack of appetite etc.
he is now on 10ml of lactulose (stool softener) twice a day and 5ml - 10ml of Sennakot once a day to make his bowels move. He insists he can't "feel it" when he soils and I feel so terrible for not realising the problem earlier.
Now he occasionally will say he needs a poo and goes to the loo, usually his pants are already a little bit dirty. Now if he sits down quietly with a funny look on his face or farts, it is off to the loo and usually a poo follows and the sheer amount of it (sorry TMI) shows how long it must have been there.
Sorry for long post, to summarise, I do feel we are progressing slowly, so I think it is try another doctor to get the diet and drugs right, loads of fluid, and back off the pressure if you can. I do wonder how all this will affect DS2 in later life!