Sorry to hear about your roommate's problems starting to become yours. I have had my brush with negative influences/dead weights in my life. Here's a great story:
I had/have a friend who lived with me at my parents house in highschool after his parents kicked him out of the house. This same friend came to the subway (fast food) I was managing after he got fired from his job at a dealership for getting caught doing side work by the big, big, boss. I had to fire him for stealing money from the cash register. Stealing/eating food, ok I'm not too worried about, but stealing money from the drawer in unacceptable.
It's really bad since the company knew he was my friend, and he ends up being a thief. And of course, I caught him doing it once on my own, but then one day when there was another cash shortage, and video of my 'friend' taking it. The store was broken into sometime around this time also. The bad guys made out with hundreds of dollars, and it was obviously an inside job because they knew where the cameras were, even the ones in the back of the restaurant. I found out through the grapevine that it was he that got a friend to do the dirty work.
This guy, like yours, buys expensive crap food, dip tobacco, MJ, and alcohol alot more than he can afford.
The way I understand it is that some peeps never develop their frontal lobe and continue to operate on the toddler/teenage level hind brain. Some of us grow out of our wholly impulse-driven, pleasure seeking, risk taking programming and open up to our empathatic, self-control thinking.
I usually give people a little slack, until I give up on em and cut them out of my life. Like that one guy, other friends still hang out with him, so I see him occasionally. I'm always friendly/civil, but you won't catch me inviting him to hang out ever!
As far as deadbeat roomies, I've only ever encountered that along with other responsible roommates so the damage was spread out and not so bad. It was more of a "well at least we don't have do deal with their shenanigans anymore, plus I have a great friend who would love to take their place!"
sorry this is getting so long!
Right now I just started a lease, it's $675/3bedroom (2.5 bedrooms really) Most of my friends are planning to live on this bus converted to RV over the summer as they go from music festival to music festival. I can just see it now, people running out of money and patience, too many people living in too little space... and hopefully that's the worst of it.
Anyway, this place is a steal and I was quick to sign the lease. I figured some friends would be interested when they get back to town with nowhere to live. Well, time will tell, for now I'm soooo broke, even borrowing some money from my dad at the time. So I'm right here with you in the stressing over living and billing arrangements. I hope it turns around for you. For me, I know I could find random roommates, but I think paying alot extra for awhile and being completely broke is a sacrifice I'm willing to make to ensure I get my awesome friend roommies in a few months.
Sorry, I don't really have anything good to say except I kinda understand and irresponsible people stink! Maybe u could scare him by telling him you are planning to sublease the rest of your tenure on the lease. This might get him thinking about what it would be like to actually have to pay his fair share, and how he'd get kicked out after not paying. Sometimes people don't think stuff through, maybe he hasn't even considered the possibility of him having nowhere to live.
Ok, well I gotta end this thing, I wish you the best, keep us updated! Ya never know how the universe works

I just had a deadbeat friend (well, his responsibly gf) win 50 million dollars in a lottery, that's 30million takehome. A curse if you ask me, but hey, maybe your friend will come into money and actually pay you back
*hugs*