There isnt too much you can do to force the poo issue to be honest ime. You can bring out the big gun bribes - i used lollipops for dd2 as had (and still have) such big probs for dd1. It only took 2 or 3 which arent too great for teeth but it worked a treat- in less than 3 weeks too ! If it hasnt worked whomever is looking after your ds will just have to deal with the mess!
I'm going to watch this with interest as I'm struggling with ds2, he is fine with no pants on , but as soon as I put underpants on he wet's , he even went all night last night with no nappy on and stayed dry and took himself off to the potty this morning, but within 5 mins of undies on .....accident
my dd1 is 4 and still asks for a nappy for a poo. so no answers here i am afraid. though it shows a certain amount of control you might find he just waits for night time nappies and there isnt a problem>?