Natural consequences and consistency are the keys to making sure your child is safe and avoiding a power struggle.
My guess is that your 14 year old enjoys the autonomy that riding his bike to school gives him. If possible, set up a less desireable alternative way for him to get to school until he agrees to wear his helmet.
"because you are not safe without a helmet I will drive you mysef before work in the morning even though you will have to arrive early. when you wear your helmet you can bike again."
This sets up a scenario in which the consequences flow naturally from his own actions and avoids a "because I say so" situation.