You
can make him wear it. You're the parent and you make decisions for him until he's 18. That's your job.
Tell him it's necessary to wear the helmet to ride the bike. If he again refuses, remind him it's your household and he has to abide by your rules. If he again refuses, run over his bike with your car! That should make it clear to him you're serious. That's what I'd do - but then I have no problem with confrontation!
Sounds like his pop's overdue on some corporal punishment with your kid. He needs to be reminded. Pops sure should be backing you up on this - that's his job.
I guess the question is can you live with him defying you over something serious like this? If you can, get used to it 'cause he's only 14 and it'll get MUCH worse!
Why must he ride his bike to school? Make him get rides from his friends. Make him walk. 8 miles each way will surely build character! Get him some running shoes!
Take him to the head trauma ward of a hospital and let him see what happens to other "invincible" 14 year olds.
You're in a power struggle with him. Stop it - you can only loose. You must reaffirm that you're the boss, not him, and have very dire consequences ready if he disobeys. No idle threats - serious action that you will do. Grounding, loss of allowance, loss of video games/TV/stereo - whatever he loves and make it big.
You say he's bigger than you and hot tempered. That implies he may hit you. If he does, call the cops and let them scare the **** out of him. Someone has to remind him the pecking order in your household.
As Barney Fife says, "you've got to nip this in the bud!"
good luck!