Do you have a particular teacher in mind? A friend of mine describes the teacher's characteristics exactly without actually mentioning her name. It seems to work well for her.
I would also suggest discussing it with his current teacher if you have a good relationship. In our school, the current teachers and the next grade teacher get together to decide the placement so, while the letter may not be influential, an advocate in the decision-making process can be. Again, it depends on the school's procedures, though.
In our case, I shared my concerns with my second ds's kindergarten teacher because my first had a nightmare of an experience with one of the first grade teachers last year and I absolutely never want to see that woman again much less subject another one of my children to her. We don't even get to write a letter, but I just talked with his kinder teacher about the situation and am hoping that they respond accordingly. Fingers crossed on that one.
