How to persuade a guy to not to be jealous?

Okay I know it's kinda bitchy to hear. But hey, I've done my homeworks and revision so I guess it's time to ask for a help here in IIN. Lol. :)

I have a special friend, let's name him "N". We're classmates now in college. We aren't couple but we have this mutual understanding for each other. Our birthdate are same, our routes when going home are same, and we sometimes think the same. We're like soulmates but we have no commitments.

I also have suitor, let's name him O. O likes me since the first week of our class and N knows about this. O and I are good friends and we always catch up on school stuff and music.

But these days, N is kinda rough to talk with. He somehow changed and he's getting too clingy to me. He always hug and hold my hand in public as the day goes by and I'm feeling a little bit weird about it until N confessed to me that he's terribly jealous of O. That day he started to drink alcohol and feel so hopeless to himself. I told him there's nothing to be jealous about it but he can't control his feelings. I'm feeling a little bit scared about this and I don't want him to be changed because of this. He's a good guy and good friend and I don't want him to poison his mind or feeling. What should I do?

(PS: Sorry for my retarded English. I really need an advice.)