Overtraining can be psychological and/or physiological. He's not going to gain intrinsic motivation to box if he's forcing himself to do it. When you love something you want to do it, that's intrinsic motivation. If he's doing it for something other than the love of doing it then that's extrinsic motivation and he'll most likely not get the best out of himself and give up completely at some point sooner rather than later. He needs to vary the intensity of his workouts, the volume and get some quality rest to break up the monotony. That's what's going to keep him motivated, not 'toughing' it out or trying harder. That's stupid ass old school bollocks. To the TS- have a good rest and forget about boxing, if you're not enjoying it you're not going to do well. When you get back to the gym pace yourself and work on different things, vary your workouts and keep it interesting. Get sleep and eat well, it'll push you towards overtraining if you're not getting quality in those areas. Yes, mental fatigue can be a sign that you are starting to overtrain. Listen to your body and be honest with yourself.