Physical therapist here, is it a dislocation of the knee cap (i.e. patellar subluxation/dislocation), or of the actual knee joint between the femur and tibia? Whatever the case, it would be a good idea for him not to only continue on with physical therapy, but to also consult an orthopedic doctor. It's not uncommon for people to become more susceptible with repeat injuries, or perhaps his anatomy is prone to this particular type of dislocation. Whatever the case, it's happened more than once and it's significantly impacting his physical and emotional well-being (and overall quality of life) that it he should be seeking help from a medical professional too (PT is an allied-health profession). Not to mention surgery may be indicated to prevent further dislocations in certain circumstances.

He will need to understand that the human tissue can only repair and regenerate at a certain speed, and PT is just there to help a patient make an early return to function/activity, and not to help heal any injured tissues. Similarly, muscles can only strengthen in such speed that rehabilitation is something that requires patience, and diligence. Make sure he explains what his specific goals in physical therapy are, so that the PT can better understands what he needs/wants to return to doing.

Physical therapy is also a place to learn, he should try to utilize his time with the PT to the best by asking plenty of questions and learning from this experience so that he can make changes to his everyday life while he recovers to prevent further re-injuries, and make a better recovery. He should implement these changes to make short term goals that are achievable in his everyday life in a short time span (e.g. pick things up from the floor without causing symptoms, climb stairs normally using alternating steps, etc.). Goals that are geared around his everyday tasks and responsibilities will probably have a greater impact on his quality of life. Together with this, have a long term goal set, this will be geared towards returning to full time activity whether that was work or physical activities. Goal setting is a crucial part in physical therapy, try reading up on SMART goals to better understand what parameters are important to consider when setting a goal.