Have you explained to him this specific reason that you need him to answer? In case of emergencies? If you haven't, you should, and make it very clear that if an emergency occurs he must answer, and should always answer.

If he still fails to respond to you, then make an ultimatum. He uses your car a lot? Well, if he can't even be bothered to respond to you, maybe you shouldn't be bothered to let him use your things! If he gets what he needs from you within reason, You should get what you need from him that is within reason. And wanting him to answer your messages is entirely reasonable. Honestly, if he is not at work or driving he should answer the phone. If he can't, he should call you promptly when he is able to do so. It is just basic courtesy.