Well, I guess I would handle it a little differently myself. Instead of focusing on what you don't want him to do, focus on what you DO want him to do. Think of it this way. There are SO MANY possible wrong answers. If you are very clear about what the right answer is, AND TELL HIM WHEN HE GETS IT...now that would be a little easier, wouldn't it?
Get a picture in your mind of what it is that would make him your "perfect foal." You mentioned that he really has a sweet spot for scratching. That's great! That is your reward spot. Let's start with the biting, since that seems to be the biggest hot spot for you at the moment. Where would you like his head to be when you are around him? Facing straight forward? Nose on the ground? YOU decide. Just make sure your selection has his nose somewhere away from your body.
I use a marker signal that is my yes answer cue for the horse. I use a "tongue click" sound, where you pop your tongue down off the roof of your mouth. It is a pretty distinctive and sharp sound. You want to have your foal associate the marker with the reward. So you can charge that sound by making it, then giving him a quick scratch. NOT TOO LONG. You don't want to encourage him to perform mutual grooming! Just 1 or 2 seconds worth of scratch should be good enough. Once you have done this about 25-50 times, he should start to figure out that when he hears the marker signal, he is going to get something desirable (a scratch). Now you can start thinking about rewarding behavior.
Watch your foal as he moves about. Keep out of his reach, but be ready to mark and reward the very second you see his head move into the EXACT SPOT you want it. That really is the purpose of the marker...it has the ability to mark a very precise moment in time. The marker is strongly associated with a desirable thing...scratch, food, even to be left alone! Depends on what is motivating to THAT animal.
In the case of a biting foal, I would probably prefer that he swing his head AWAY from me to get his reward. And yes, I would teach a biting horse this process using food. I have done, very successfully, much more successful than ANY other method I have used.
For rearing, you will want to teach him an incompatible behavior. For example, he can't rear and lower his head at the same time. Head lowering is pretty easy to teach. You might also want to teach him to touch and hold on a stationary target.
Here are some good resources for you if you decide to pursue this route:
http://www.theclickercenter.com
http://www.clickryder.com
http://www.equineclickertraining.com
http://www.canadianclickercentre.com
Your little fellow just needs to understand what is expected of him. Flicking him probably won't build the kind of relationship you are really looking for, in my experience.