Please help me? I don't know how else to tell him?

My boyfriend won't wear a helmet on his bike because it's not 'cool' and he won't listen to me or my mum when we tell him what could happen to him. I'm sick of him thinking he's invincible and I don't want to seem like a safety freak but my mum is a nurse and she's seen first-hand what head injuries can do to someone's life. I don't want him to think I'm trying to control him but it's only because I care about him and I can't lose him whether it's to brain damage or death. I know the risks of this are low but we live in quite a hilly area and I know people who have gone over the handle bars and have their helmets crack in two. They were lucky that it wasn't their head that cracked in two. How can I make him listen? My mum has even offered to buy him a helmet.