Next time he opens his mouth, shove a leafy green in it. Collards are the fave of my igs, followed by turnip and then mustard greens. Thinly sliced (no wider than their heads) yam or yellow squash is another fave. Once you get something in its mouth, you can piggy-back more food - they pull the food toward the back of the mouth with the tongue, so it's like a little conveyor belt. When the first piece of greens is almost all the way in, place another one on top of it and they'll keep chewing. They do love bananas, so that could be a good way to get it started and follow up with the veggies. Some folks will shred the greens into tiny pieces. Mine big ones enjoy tearing them off the stem. I usually tear the greans into pieces the size of their heads, some smaller, some larger. It's easier for them to get a hold of than tiny little bits.
I have some short vids of my big girls eating on youtube:
YouTube - johnnieranger's Channel