IMO escarole is easier to introduce than kale.
The other method is to prep his normal food as usual and present it on a plate. Next to his familiar food, make a medium pile of head-sized pieces of the green you want to introduce. This way he doesn't get a separating-out habit. Over time he will become familiar with the sight and fragrance of it. Often they just start eating it on their own. This could take months to work out, and you have to present the new offering every day.
As a last resort, if he has a will bigger than yours, you can decide he's ok the way he already eats. I advise you to feed supplements in very small doses, like a tiny pinch every other day. You want to avoid hypervitaminosis which is damaging to the liver and/or kidneys(I forget which) over time. Damage doesn't show until it's too late and then it's an incurable disaster.
This article has a very good section on Picky Eaters: http://www.anapsid.org/pdf/icfs.pdf
Scroll way down.