So he's capable of holding it.
You don't want to get into a control struggle with him. This is a "pick your battles" issue. My child development prof told us there are things we can't force kids to do, so making them into a battle is a lesson in futility.
We can't force them to eat.
We can't force them to fall asleep.
We can't force them to use the restroom.
What we have to do is create boundaries that makes it preferable for them to follow the rules. For instance, "Five bites of this healthy food, then you can have your choice of food." If they choose not to eat those five bites, fine, they don't get the preferred food.
"You don't have to sleep, but you do have to stay on your bed doing a quiet activity such as looking at a book."
For potty training, we've used a few tactics in my daycare and parenting experience. They might create more work initially but tend to work in the long run.
My kids had naked time and learned to use the potty, that and the cloth diapers that made having a mess in their pants uncomfortable.
We've also had parents go shopping with their kids for "cool big kid" underwear. The kid picks out their favorite characters. You tell them, "Your job is to keep your Superman underwear dry and clean. When you go in them, we will switch to these underwear." Then get the big, thick ugly training pant underwear for him to change into.
If you notice he tends to go into a corner or other secluded place to release his BMs, watch him as close as you can and when he does that, stop him and take him to the potty.
Finally, for some kids that like the control, we've had them change themselves. We've only done this when they show they have knowledge and control of their BMs. Yes, we provide assistance. But they have to pull the pants down (we have changing paper for that part) and dump the poop in the toilet- because that's where it goes. Then bag up (or at home I suppose put it in the wash) the dirty underwear and help wipe themselves down. For children who are trying to control or think it's faster to just have you change them, this time consuming process means they have less play time.
Finally, consider the fact that if he's going on the big toilet, he might be afraid of it. If he associates the pain of the constipation with the toilet, or even with falling in, it might contributes to him resisting sitting on the potty.