If a friend asked me this, I'd be saying 'They've given you a key, go and check'.
If you get there and there's clearly something wrong - including after going around looking in windows - you can call the police and tell them. If not, you can get in the house without having to break in which they wouldn't be able to do.
If anyone asks, you're a friend, he gave you the key, and you pop round every week. You've already called the police because you haven't heard from him in a few days without getting them to do anything, so you went over.