I am far from an expert with Hammies but i had a similar thing with the bed when we got ours, also black and white Syrian
Try tearing up some toilet paper and put it in front of his house, it may encourage him to drag it into his house for nesting and sleeping. It took a couple of days with our Frankie, but eventually after getting used to his surroundings etc he moved in his bed and was comfortable. Try keeping him on the same food as he was previously on as well for the time being, changing it could stress him out more. You could gradually try changing it as and when he gets used to you more.