Ain't that the truth! Nothing like trying to scramble to go back in time, looking for `proof'.
The best evidence, is something that you don't have; a joint lease. However...there is another option.
Do any of his neighbors know that you've been there all this time? If so, ask one of them (or even a couple of them) to write a letter stating this. The letter(s) would have more credence if they were notarized, which is a bit of a pain (and expense). Ideally the landlord could also write a letter, which would be the best option.
Do you have any mail that's been delivered to you (or both of you) over the span of your visit? The older the mail, the better.
You can also see about being added to his bank account. I was added to my partner's account without any problems at all, even though I was a visitor in Canada. Some banks are easier than others (we bank with Scotia/Bank of Nova Scotia), but I hear that TD is easy to deal with as well.
Things like a joint cell plan, being added as a beneficiary to his life insurance, etc., are also popular choices.