Wow, 26 students is a LOT of kids for a kindergarten class. I'm a teacher in BC, Canada and by law 22 is the max, and I think that 22 is a lot.
I haven't read the other posts so I'm sorry if I'm repeating any advice someone else has said, but I would suggest that for the next few years at least, your son will probably be ahead of his peers academically. This means that now is a great time to teach him how to extend his own learning and challenge himself. In my experience, truly bright children are never bored...unless they are discouraged from exercising their natural curiosity and desire to learn through play. I would ask the teacher about her program, and if there are any opportunities in the classroom program for extending his learning. For example, if they are doing waterplay at centers, might she give him an opportunity to conduct his own experiments (eg sinking/floating, or which structures hold the most pennies and still float on top of the water...) and then perhaps he can write/draw a picture of his experience at the water table.
I don't ever think it's too early to help kids learn their potential to be self-directed learners, and it sounds like your son is slowly slipping into an apathetic role and he needs to realize that he is in charge of his own attitude and therefore his own potential for enjoyment! I mean, I could say, "I'm bored, this job is too easy," but instead I choose to challenge myself and take new opportunities to make my job interesting and fun, kwim? We are not consumers of education, we play an active role as learners...therefore if I'm bored, it's my problem and I need to take action! Yes, even at 6!
There is a student in my class currently, a grade 2 girl, who is has so much potential but is convinced that everything is a "review" and her grandmother (who raises her) is convinced she's bored because she's not being challenged. The thing is, I provide lots of opportunities for her to extend her learning, but she doesn't take them. With writing activities she does the bare minimum required, she turns down books I suggest that I think would stimulate her, and she chooses the easiest strategies for math rather than challenging herself to come up with new strategies or find more than one way to a solution. It is the saddest thing! She could go so far but she's totally a "lazy learner" who would rather be smug and say she knows everything already! I would hate to hear that your son becomes like that, because it's not a happy place to be.
Anyway, definitely talk to the teacher and be proactive, and involve your son too!