I want to tell my quadriplegic friend to be grateful for what he's got
Why would you tell him that? Was he complaining a lot?
No, he told me he wants to keep going by always trying new things, challenging himself.
All things considered, I also think that your friend it's still lucky that he's alive. But I think telling that to his face makes you look like a rude person, and you shouldn't outright tell people that they need to be grateful especially if you haven't had any idea on how they are dealing with things while being disabled. How about you ask him if he needs help with anything, and let him know that you're there to help him if you needed to.
Who are you to call me rude, mentioning feeling lucky, my friend is right, he deserves to feel alive and believe in himself and his life. That's what I need to tell him.
I think it's great if you want to encourage your friend to feel good about himself. Just make sure that you are always considerate because your friend is dealing with a very unique situation.