pooh leaking in 3.5 year old

[Spanglemum]

NOt sure if this should go in here or children's health. My son is very reluctant to be potty trained and has had problems with constipation last year which required Mocovium (sp?). I think he is past that now but he still seems to be leaking poo. If I let him run around without a nappy on I have to keep wiping his bum. Also I don't think he ever really empties his bowels, just does enough to be comfortable. Can anyone help, he's supposed to be starting nursery in Sept where he has to be potty trained.
With wees I think he can hold on, but refuses to go near a potty and has only gone once or twice when sitting on the toilet.
Basically it's taken us ages just to get to the stage where he will sit on the toilet and try.
I realise there are several problems here but if anyone can help I'd be very grateful!

Spanglemum