Potty training a newly 3 year old.

My son just turned 3 last month.. he would be 100% potty trained if he would just poop on the potty. But anyways, he's been holding it for 5/6 days now even with a pull up on.

His doctor said it's okay as long as he isn't constipated. Which he's not.. But it can't be okay to hold it for this long. Any ideas to get him to go? He's very strong willed and is 'scared' to poop on the potty. Even trying to bribe him with a new hot wheel each time he poops.