Fenrir's behavior is likely due to his upbringing. You have to keep in mind that Fenrir was betrayed by his caretaker (Tyr), taken forcibly from his family (Loki) and locked away as punishment for being "scary" to the Aesir. He's got a lot of pent up anger due to this. He also has a ravenous hunger, and is beyond control due to his appetite.
However, that's not to say he isn't a deity you can learn from. He's one of the better deities to try and understand if you wish to learn more about your own primal, beastial side and the "darker" side of your persona. He is very much chaotic and uncontrollable. He will never be a friendly deity. He does not much care for offerings because he does not much care for anyone or anything else other than him and his needs.
Like suggested, you could leave him an offering outside. I have a tree stump I use. If you feel like leaving an offering, I suggest something you've hunted (skins of a rabbit, etc) or meat (bloodied and raw). It's also been said that pain, negative emotions, anger and blood are acceptable offerings to him. But don't expect any kind of show of gratitude or even mention from hiim just because you do something.