S.F.
You could try getting him to go on the portable training potties. That worked better for my kids. Sometimes they would get busy playing or watching TV, and they just refused to leave what they were doing. The babysitter would put a potty off to the side of the room and remind them to go. She had another one in the bathroom as well.
I used stickers to reward my kids for trying. That worked well for one son. The other I used several different techniques. Finally, I found that I could reward him with a small piece of chocolate or a sucker for trying to go poop, another candy for going poop, or maybe a candy for going and flushing the poop down the potty. I suggest start on smaller goals and work up to going poop on the potty.
Maybe if he's wearing a pull-up, you can ask him to tell you immediately after he's gone and reward him. Perhaps that will help him realize when he needs to go...?
I also had my boyfriend go to the potty with him. The two little boys and him would go together like a big male bonding thing. I instructed the babysitter to have her older sons help my boys out or talk about it like he was a big boy for going.
I offered several rewards for going such as the park, zoo, playdate with a friend, taking out the sleds to play in the snow, going to the Dollar Store to pick out a toy, etc.