I think %15 is a good number to go by. so nothing bigger than %15 of the snakes body weight, I'm sure someone on here can verify that number. Maybe try switching him up to rats depending on his weight. If he's used to being fed live and hes never had a problem with it for the last 4 years, keep feeding live. I guess you won't really know for sure if he'll even take f/t unless you give it a shot.