I have trouble believing that he didn't shed for his previous owner (6 months?!), but if that's what you were told, oh well. He sounds like he is probably still healthy, and it is not unusual for a male his size/age to be off feed right now. Just keep weighing him every couple of weeks, make sure you are heating the F/T enough, and wait him out. Oh, and keep him in that 20gal instead of worrying about trying to upsize him when he obviously likes the smaller space. Good luck!