One mom I know got some diapers in a much smaller size and then pointed out very dramatically, "Oh, my! You are such a big boy that your diapers don't fit you any more! I guess it's time to shop for big boy undies!"
And then of course if he insists on using the too small diapers they won't be comfortable for very long.
What about school? He can't go to kindergarten in diapers, but I guess if you're home/unschooling that argument won't work.
Something else to consider is responsibility. By 3, our kids had jobs to do, like cleaning up their toys at night before bed and helping set the table. What if he's responsible for dealing with his diapers on his own, with some help for the poopy ones? Show him the diaper pail, where the wipes are, and remind him to wash his hands when he's done. And set all that up in the bathroom, of course, so he can choose to use the potty or deal with his diaper on his own.