I think that you need to address these separately - by putting them together, it will seem like you're making an excuse or justifying it. He has no reason to act like he does, IMHO, no matter how bored he is. I say that as the parent of four gifted kids- all of whom have been bored at one time or another. So, I would first address the behavoir - what are the school rules, how are they being enforced, how can you and the teacher work together to change his behavior.
Then, separately, I would say, "I'd also like to talk to you about challenging ds more." Try to bring in some concrete examples of his abilities: books he's read, stuff he might have written at home, any testing, etc. Talk about what you'd like to see happen-"could ds have a harder spelling list?" "What level do you think he's at in reading.. I think he could be reading more difficult material." Most teachers are pretty responsive. I've been able to get some level of differentiation for all my kids.
If you can, look on the district website for any information about enrichment/differentiation. This might help you support your case.
I understand that your son is bored, but rolling his eyes or putting his head on the desk are unacceptable. If you start out defending him or excusing him, the teacher will NOT be receptive!
Theresa