I have a fail proof method for animals that don’t want to eat. I don’t make them! I always start by making sure my husbandry is dialed in and the snake isn’t sick. Beyond that if I want a snake to eat something then that’s all I offer unless the snake starts losing a lot of weight, which has never happened for me. Whether that’s transitioning from live to f/t or from mice to rats my system is the same. In this case your snake has eaten f/t before and is eating live now albeit hesitatingly. I agree with BCR. Wait 2 weeks and offer a f/t rat pup. If he doesn’t take it from tongs leave it overnight. If he refuses twice then space the feedings out to every three weeks. If he refuses twice space the feedings out to once a month. If he refuses this feeding then offer him one very small live prey item which he’ll certainly eat. Then start the process all over. My stubborn critters usually give in after 3 months except for those darn BPs, they can go forever. I doubt your burm will take very long to get on track. The hunger ALWAYS wins.