Well, essentially it's his problem, sorry to say. I know you feel a little responsible (and who wouldn't?), but are you really? I think the soonest you tell him, the better. Offer to help him find another place... but really, you have to live your life.
We can't be responsible for everyone in our lives. Maybe his family could assist? Or is there some govt agency that can help? Helping him explore all options could mean a lot to him and actually help him, also not to feel abandoned.
My 84 year old mom was paying $2250/month and then a friend told her about the Kiwanis Lodge. She applied and got a larger room for $450/month. There are agencies out there that might be able to help your roommate.