I am an absoluty newb buy I would suggest maybe trying live if that's what's he was eating previously. Try moving slowly towards f/t for example:
When I got my pastel she was on live rat pups and I started her on a few love mice, then to fresh killed, now she eats f/t like a champ! But every snake is different. Just have patience with the big guy I'm sure he will come around. I remember reading how long pythons could actually go without a meal, and people freak out much too soon. bat of luck my friend