If you wanna feed your snake live you can try live. if you wanna keep it on f/t the don't try live especially this early. 3 weeks off food is like a couple hours for a human. With his age and weight he could go a couple months before you would even need to start to worry. As long as he appears healthy and isn't drastically lossing weight and you have the setup done properly you don't have anything to worry about. Most people freak out way too easily when their snake goes off food and don't understand that it is normal and they tend to start trying to much to get the snake to start eatting again an that can just put the snake more off food.