Don't force it. Let him learn to scoot - perhaps not on a slope, that might be too much, just on a flat bit. If you can, just leave him sometime to get on with it. Your presence, and keeness, might be intimidating him a bit - he says he's afraid of falling off, maybe he's also afraid of looking stupid and disappointing you?
So what if he doesn't learn just yet? I didn't have a bike until I was 10, and then I learnt, slowly. Same thing with swimming - never mastered it at primary, then suddenly it came right when I was 11. I was late learning to talk too - because I didn't bother to say any words until I could come out with a meaningful sentence, Well, apparently, my first clear words were "goodness gracious me!", which I must have heard my Dad saying.
Now, I can cycle, and swim, and talk the hind legs off a donkey. But not all at once.