D.T.
The simple straight-forward technique is best. Call the school to get her voicemail during the day and request an appointment to discuss his progress. Teachers pretty much expect parents to be involved and want to know what's going on with their kids (at least all the teachers I've met). Then when you meet with her, explain your side of the situation. Tell her that he's been complaining of being bored and doesn't feel challenged and then work together to find a solution.
When I was kid, I was never challenged in school but it was a small school system and no advance classes of any kind. So I continued to do the easy grade-level work and my parents taught me more advance level stuff at home. I wasn't as bored and it really helped... I could zip through the school assignments in no time. Maybe something like that could help him.
Schools these days are better about that stuff. Find out what your school has to offer. Our school has a Challenge Class for kids who are bored with their grade level in most subjects. All the other kids actually split up for reading, math and science - just like a jr high would. Meaning of the 4 1st grade classes, all the kids have math at the same time in separate roomes... one room for the most advanced, one room for least and the other fall inbetween. There's not a huge gap between the levels and kids go up/down all year long and it's not a big deal -- just depends on how they grasp a particular subject. So I know my kids will be challenged (my 4 year still has another year of preschool next year and is reading kindergarten level and doing 1st grade math).
Good luck!