There is a lot of things you could try. Try using a frozen thawed fuzzy and leaving it overnight. Another tip I have used is to take a f/t fuzzy and wash it with plain Dawn dish soap. Rinse thoroughly and then offer it to him. Sometimes the sand and rubber boas are put off my the domestic mouse smell.

If the snake has only refused a couple of times I wouldn't worry. They can go a while without eating before you start having real problems.