I tried to share my thoughts and experiences as a previous motorcyclist. The tiniest malfunctions can hurt you badly, with or without a helmet. Then you must consider road conditions and other motorists. It really isn't practical to introduce another worry. The rider must have a certain amount of good sense and responsibility, otherwise its just a matter of time. No gadget is going to make the operator think better, it something that needs to be learned.
How about more of a tattler type circuit. It records whether or not the helmet is being used, temperature sensor perhaps. When you check it, and the rider hasn't followed your helmet rule, put a heavy chain and strong lock through the wheels, let him walk, or bum rides from friends for a while. Always, more fun to ride. even wearing the helmet. Eventually he will learn to be safer.
Sorry we can't just design this circuit for you, or help you mess around with the wiring (we don't know what motorcycle, and some are very different). Never heard of any 'hair' sensors, or any ideas on how that would work. But trust me, the worst things happen on a bike, when you need to react instantly to something unexpected, and even then it might not be good enough. You only get on chance, thinking isn't an option.