I'm not a lawyer, so this isn't legal advice. You need to talk to a lawyer. However:
If you are not on the lease, then you have no contract with the owner/landlord. And it doesn't matter where you receive mail.
Nor could it be argued that you are subleasing. First, all real estate agreements must be in writing to be enforceable. You don't have any document to establish that. All you have is a verbal agreement. At best, it's your word against his. Technically, from the standpoint of the lease, you're probably a "guest."
That puts you in a very weak position. However...
You don't say what the term of the lease is. (Year-long? Month-to-month?) That doesn't help you much, but if it's a year's lease (or something that extends beyond January 15), then your roommate will be in violation of the lease.
One other point: You say you've been paying your rent to your roommate. Has he been paying the rent to the landlord? Or has he been keeping it for himself. (I kind of guess that's what's been happening, and he knows he's going to be evicted.)
I also don't know what your relationship is with your roommate, or whether you might expose yourself to any harm or danger by resisting him. I wouldn't want that; you have to be the judge of how he might react to the following advice.
I'd suggest you contact the landlord. Say, in your own words, "Mr. Landlord. You know we moved in just 6 weeks ago. I like it here, and I'd like to stay. However, as you also know, my roommate is on the lease and he wants to move out on the 15th. I would like to begin leasing the property from you on that date. That way, we wouldn't have to move and you'd still have a tenant here paying rent. The alternative, unfortunately, is that he moves and I go with him."
So: Contact a lawyer. And consider whether your roommate would be OK with you continuing to live there if he moved on.
When I take a lease listing, I usually post on as many internet sites as possible. If I had to pick the site that provides the most traffic, it would be Craigslist. Not sure if you are new to finding tenants, but, if so, send me an email and I will give you a couple checklists to use.... more