Revisiting Mendelssohn Violin Concerto

My son has learned Mendelssohn Violin Concerto 1st mvnt from his previous teacher about a year ago. Since he's entered in a local competition with this piece, he's been working on it all this time, so he can be prepared to perform with the orchestra as a part of his prize. Since his last teacher moved out of state, he's been working on it by himself for several months until his new teacher, Mr. K gave him a new bowing, fingering. Now, at his last lesson with him, he asked my son to play it. The first page went quite well. He fixed his intonation, articulation and dynamics, so he asked him to play the rest. This was quite different from the first page. I know his tempo was unstable in some places. He tends to rash at times where he needs to keep his tempo. His sixteenth notes were not clean. All these areas, I was aware, but he doesn't want to listen to me. This time, his teacher confirmed it. He thought he was doing well, but now he is in shock. Until next lesson, he was ordered to play it by sections and absorb the dynamics, clean up the sixteenth notes and study phrases with a metronome, of course. For a twelve-year old boy who wants to play everything fast, this is a pain-in-the-neck practice regime, but I know he needs to. At the same time, I am realizing how difficult it is to master such a big concerto. Although he doesn't realize the benefit of slow practice yet, I am certain that when he's done, he will see it himself how much he'd improve.