ec_bb is right that it is always good to try to avoid confrontational approaches. It is just so hard to strike that balance btwn not being confrontational and letting them do nothing and being so confrontational as to make them angry and defensive.
I think that I'd start with what ec_bb suggested and "check in" to see what is currently being done. I'd then take that info to request, in writing to the teacher with a cc to the GT coordinator, that something more be done. I'd acknowledge that you have a compliant child who is presenting differently in school than at home, but that the distress he is expressing to you at home is causing enough concern that something more needs to be done.
I really think that asking for subject acceleration is a good option and ask that they use the Iowa Acceleration Scale to assess him for acceleration. It can be used for both subject acceleration assessments and grade skipping assessments. It would also require them to give him some sort of an ability test, which would be good to have on file.
The only fear I'd have there is if he doesn't come out super high on a group ability test. I have one whose individual IQ scores are astronomically higher than her group ability test scores were and she's my compliant one who never appears to be having issues at school, but who is a different creature for me. It makes it hard to advocate when the kiddo both appears to be doing fine to the teacher and when the measure they are looking at doesn't indicate HG+. We had to have multiple IQ tests given, which all showed gifted+, before we got them to agree that the first set of IQ scores weren't due to "good guessing."
Might you have the ability to have his IQ tested privately? That might be another option to support a request for subject acceleration. It is also less risky b/c the school only gets to see the scores after you have and if you then choose to give them to the school.
