He might be stressed. Did you give him a week or so to calm down before feeding? Or handling? Baby boa's often use hides as well so make sure he has on of those. You may want to try live until he is an established feeder for you and then switch to FT. Give him a rat hopper or mouse that is the same size or only slightly larger than his widest part so when he eats it you will see a small bulge.