I would try live prey, if done properly live is just as safe as F/T or P/K. What I would recommend (and I am far from an expert) is to get some tongs and hold the live rat by the tail and let it be just out of reach from the snake.

This way the snake has the chance to strike and feed and the prey can't harm him/her. Your temps and humidity seem fine and he should have had time to adjust to the new environment. What temp is the food when you offer it? They should be nice and hot (but not cooked) and if needed warm em up real good with a hair dryer. But I would try live for the next offering of food.