i have a few ways...
punch the midget works, as said above.
if he can do backflips, start him doing a straight backie, then make it a back 180, when he gets comfortable spinning a 180 with a backflip he should be able to make it a 360 backflip. Once you do that you're basically there. He will know the feeling and it's just a matter of messing around with it until you're doing cork 7 not back full.
The other way is to start doing 7s and just keep trying to dip your shoulder more and more. at first they will just be weird looking 7s, but eventually he'll start to feel where in the spin you need to dip your shoulder and it will get more and more corked.
I learned cork 7s when i was very very young just by kind of hurling myself backwards and spinning. Most of the time i just landed on my back/stomach/head, but it never hurt. After about a day of doing that I bounced a little higher and just tucked my body more. I didn't land but i felt myself coming around and that's all it took, I had it stomped within a few tries.
I would stick with the other ways I described, but when all else fails honestly just start throwing yourself backwards in different ways until it feels right, then go for it.