How do you deal with your SO's bad mood?

I've been with my boyfriend for a long time now and I still don't know how to deal with his bad moods. He gets irritable and negative in attitude. Just completely closes off.
When I'm in a rut or in a shitty mood, I'd like to find empathy and hear words of encouragement. However, none of that works with him! 
He doesn't like it when I try to empathize by telling him that I care, that I love him, and we can get through this together. Nor does he like it when I'd talk about my own similar experiences. "It's not about you", he'd say.
Then when I try to cheer him up by telling him to look ahead and that things will be better. He's just like, "You don't know that." 
Argh! I know it's all clichéd but the whole point is to show that you care and that you're there for them!
We have really different personalities and what works for me doesn't work for him. Maybe I should just leave him be and wait till he's ready to engage. But it's difficult for me to do because I worry about his wellbeing. He's the type that can keep sinking.
Any of you have any advice for me on how to comfort this kind of personality?