You can purchase a type of insulation for your outdoor hutch or you can put siding around to keep drafts out.
As long as bunny has a place that is completely covered and safe from the elements, plenty of fresh water, timothy hay and a good diet, he should be fine.
Make sure to give your bunny plenty of hay as bedding in the winter and summer months, change it regularly as he will soil on it. The hay will help keep him warm.
My recommendation is this. If it's cold enough to freeze the water in his cage, then it's time to bring bunny inside.
I'm not a big fan of keeping bunny's outdoors, but if you have the proper cage and supplies and winters aren't too harsh, then he should be fine.
As for foods. I buy dried bananas for my bunnies, it's very healthy and they love it. Of course they also get lettuce and the occasional carrot. I found a wonderful store manufactured rabbit food that isn't too expensive and is actually very nutritious. I buy it at my local pet store but I'd have to get back to you on the name, I threw out the bag and I can't remember, sorry.
If you have any more questions about your bunny I have had bunny's all my life and currently have 4. Also, my sticky (which you might have seen when you posted this question) has many helpful tips on how to care for a rabbit.
Good luck with your bunny.