Joash - I think you should try a different GP. There are medical reasons for this which should be explored - impactation being the most likely.
Sorry for the TMI questions - but when he does poo his pants, is the poo soft squidgy like at times, or is it always large fully formed poos?
Poo leakage around impactation happens a lot - and it tends to come out as soft squidy poos. The impactation stops the child from having any sensation of needing to poo - as it desensitises the rectal muscles - and the child genuinely doesn;t know that he needs to go.
There are also emotional reasons for fecal incontinence.
Whichever it is - getting angry at him, will make it worse. I;ve been there done it and got the t-shirt (well laundry basket of pooey pants actually)!
If medical reasons are completely ruled out - then this is the approach which finally worked for me:
No fuss, completely neutral when an event happens. Clean up - nothing said etc etc. No implied punishment either - like sending him to get pants/clothes etc.
When poo is done on the toilet then have a huge very conspicious bottle of lollies, mini fun bars, choccie buttons (whatever his favs are) and in a very theatrical manner pull the prize from the jar adn tell him how wonderful/clever etc he is. Do not ever mention the sweets when there is an 'accident', and do not ever mention the 'accidents' when he gets a reward.