Two Steps Forward, One Step Backward

This weekend's lesson was good and not so good at the same time. Since we can only see his teacher once a month, we try to get two hours Saturday and two hours Sunday. First day, we focused on Bach Partita No. 2, Allemanda and Corrente. Last month, my son was told to use a metronome at all time. So he's been following that instruction (or I made him:)). His teacher, Mr. K said, "First time in your lesson, I hear the metronome!" meaning his rhythm was getting consistent. That made his day! My son tends to use his bow more than it needs to be, so Mr. K reminded him to hold back a little bit. Corrente was coming along better than Allemanda. Since his intonation got better on both pieces, they started working on interpretation and accents. It is finally becoming some sort of music.
Next was Mendelssohn Violin Concerto. At last lesson, it was just slashed by his teacher. He's been playing mindlessly over and over and some of the intonation was getting sloppy and accents were ignored, etc, etc..... So he assigned his section by section a timed practice regime and told him not to play in tempo for at least a week until all the intonation is cleaned up. This month, it got better and they started working on the detail like phrasing.
After the lesson, we are supposed to go back to the hotel and practice like crazy, but we have a family friends we haven't seen in two years and I promised them we'd have a dinner together. So, we met up and had some Asian food, had a good time. We got to the hotel around 7:30pm and after a short break, my son practiced for about two hours. I don't know how focused he was but he tried.
The next day, we had another lesson around noon. We started off with Bach. Allemanda, so-so, but Mr. K was very pleased with Corrente. He told my son he almost didn't have anything to add which is a big compliment for a teacher who was very disappointed last month by my son's progress. Now, there was one more piece he needed to play for him.
That was Mozart Violin Concerto No. 4. Mr. K worked with him a few months back intensely, so he waited until now to listen. While he was playing, I could tell his face started getting tense. After my son finished, he said "We worked so much on this and I don't know what happened. I was expecting it was even getting better than before. Now, your intonation is too sharp in so many places and all the dynamics are missing." Oh, no! I don't know what to do. I feel sorry for my son for going through this emotional roller coaster, but I understand Mr. K's frustration, too. I constantly remind my son not to play through over and over, but to take time and slowly practice with the score in front of him. This pre-teenage mind doesn't want to listen lately. Mr. K told him, he can't afford having inconsistency in his progress. One gets better and the other one gets worse. He needs to slow down and take time to look at each note carefully. I can only hope all the pieces he is working on are upward progress.......