pooing in the toilet!

[Alilog]

Hi,
I'm at my wits end and need some advice. DS is 2 years 8 months. I started potty training a couple of months ago which in hindsight may have been a little early. From the outset he wanted to wee wee like daddy ie standing up. I went with it but now we can't get him to sit on the potty or toilet so all his poos are being done in his pants. He gets distressed by this but nothing seems to persuade him to sit on a toilet or potty. We've tried everything from books, to reward charts to treats and even a new Thomas the tank engine soft toilet seat. It's becoming a real problem as we've had many poo incidents when out. I don't make a fuss, just carry a lot of spare clothes but it just isn't getting better.
Help!