If you are in a car with him or out in public or even in your home, he could get busted and you, right along with
Narc-Anon, they will teach you how to
an addict and give you the support you need. You can google it and find one near you.
You ask how can you make him be honest with you?, you can't. You obviously have all the proof, other than seeing him shooting himself up. Trust us, he is doing drugs. Don't be fooled by his lies. He will lie to you a lot.