Momtogrif: Here is some advice on the mental aspects of tennis. A lot of parents and coaches talk about how winning is not that important at a certain age, that we should focus on development and winning will come later as a result of development.
The problem is that most kids have no particular goal besides winning when they step on the court. AFTER the match, parents will often say that it does not matter that they lost, but BEFORE the match the kid has been given no clear goal.
At your son's age, I was giving my youngest son ONE goal per match, or even per tournament. For example, after a frustrating point, he was to go to the back fence, turn his back on the court, take a couple of slow, deep breaths, tell himself honestly "That was a frustrating point," and then tell himself that the point is over and it is time to focus on the next point.
Before the tournament, I would emphasize how important it was to learn this, and I would tell him that if he actually did this, then the tournament was a success. And I MEANT it. When he came off the court after a win, I did not congratulate him about the win, which sends the message that all that talk about how winning was not the most important thing is a bunch of nonsense. After all, if it is the first thing the parents talk about, then it is the most important thing! Instead, I would calmly ask him whether he used the new mental technique. The answer was usually along the lines of, "Most of the time, but not always. Sometimes I let the previous point get to me." Then I would say that it takes time, but I would point out that he was already seeing the benefits.
What I emphasized to my son was that you build your tennis game one piece at a time, and there are a couple hundred pieces. His coach was working on lots of pieces called technique, and I was going to work with him on some mental pieces (and so did his coach). If he mastered one technique, we would go on to the next one. For example, when he mastered letting the previous point go, then there was the mental emphasis on not rushing his second serve, because impatience and rushing was leading to a lot of double faults. When that piece was mastered, it was on to the next item.
You would be amazed what about 2-3 mental improvement will do for his enjoyment of the game, and his results will follow as a side effect. You can also move on to pure technique goals, like your goal for the whole tournament is to practice a certain stroke or stroke pattern.
The basic problem is that coaches and parents say that development is more important than winning, but the first question or comment after a match or tournament is always about the results. In the education world this is called "the hidden curriculum," the actual things that kids are picking up on and learning even though they are not part of the official curriculum.
If a kid steps on the court and the goal is not winning, then what is it? If the kid cannot answer, then the hidden message is that winning is the thing. When you ask your kid what would make this next tournament a success, and he has no answer besides winning a certain number of matches, then he will not overcome the mental hurdles at age 11.