Trmbling knee while squatting

There could be a few reasons why your athletes knee is shaking if it is happening on the concentric contraction (way up) of the exercise ie: squats and it is as if his knee is being pulled inwards like there is a magnet pulling both knees together then it usually indicates a lack of glute recruitment, which is very common in many athletes. There are a few wasy to fix it like single leg squats, power squats (wide stance), ham-glute raise, reverse hyper are all exercises for glute recruitment. The other thing to check is hipflexor tightness, most people who sit all day have tight hip flexors stretching to lengthen the flexor will help de-activate them when squating there fore the athlete is forced to use other muscles ie: glutes. this is not to say that the flexors will not be active in the movment it is that they will be less active.

if it is just plain old shaking through out the entire movment then I would have to agree with the last guy mostlikely just a CNS issue, stabalization problems are usually more pronounced and generally tend not to just disapear thats when you have to focus on form and control.