You've got a full term.
Start by asking him to help you take the soiled pants/nappy to the potty and put them in the potty when he has been. Because that is where poo goes.
Make sure you let him see his father poo, in the bathroom at home. He needs to know everybody poos.
then ask him to sit, in a pull up/nappy , on a potty, for a poo. If he hides behind a sofa or anything to poo, put the potty there where he likes to go. Bribe, reward etc.
If you can get him to do this, you can get there.
Next step - undo one side of nappy, continue asking him to sit on the potty with it on to poo. Do this for a couple of weeks
Next still with nappy, work on undoing the other side
Then have a nappy under him in the potty for him to poo
Then cut it gradually smaller and smaller until you can swap it for some tissue.