They are opportunistic feeders and remain interested in food for a number of days after feeding. Unless this thing is an absolute behemoth, there is little if any reason to feed large rats and certainly no more frequently than you are currently. My 11.5yr old male eats a small rat every 12-21 days and does just fine, maintains good weight and muscle tone, and will still go on an occasional feeding strike. Pics would be helpful for us to offer additional insight.