What I describe below has mostly worked for our 2 year old boy-girl twins (he had zero interest in anything to do with the potty before we started and she was a little further along but not poopint). It took us about 4 days. This is not for the feint of heart/single person who hasn't been in the trenches.
Spoiler!
Pick a time when you are available to not leave your home for a few days in a row and make a production of getting all of the diapers out of the house (this is as much to prevent you from regressing as it is him). Turn the heat up and have him in underwear so you can see exactly what's going on. He'll have accidents but that's part of it - don't make a big deal about them, just clean up, take him back to his room to get new underwear and start again.
Throughout the day, you tell him every 15 minutes or so to tell you if he has to use the potty. You are not to ask him if he needs to go because a toddler will tend to say no and the point is to make it be his decision.
Anyway, if he's pee trained already, the hard part is passed because he already can use the toilet, he just doesn't. Taking away his diapers (and only place he's ever pooped) will probably force him to poop his pants but in the grand scheme, it's not that much different than the alternative. Be patient and keep a keen eye on him and when he starts straining, you scoop him to the potty. He'll probably stop and may well end up holding it in for a day or two but eventually you'll catch him mid-poop. Scoop him, run to the potty and have him finish there, even if half ends up in his underwear. If anything falls in the toilet, big deal and big treats.
This means no diapers at naps, bed time, whatever because he will wait you out if he knows there are diapers available. Prepare to clean up messes and be woken in the night with wet beds. You can help with the sheets by using pads on top of the bed sheets.
It sucks but it worked my kids and their 2.5 year old cousin, all in the same week.