hahahahaha
I feel your pain.... up until a couple years ago my daughter always asked me not to go on field trips with the class. She didn't want to share me.
The one that got me the worst was about a month into 1st grade (she'd just turned 6yrs old) she asked me not to walk her onto the playground in the morning. That only the parents of the 'babies' (kindergarteners) did that. And she asked me to stop kissing her infront of anyone.
I did get through the conversation and saw her off to school that day. Then I cried until 3 when I went back to get her.
Here's the deal. This isn't about you. I know you want to get involved, and this is YOUR experience with a kid in school. But to him it's just HIS experience at school and although you are an adult and can see this from everyone's perspective he is just a little guy and can only see it from his.
I wouldn't be the class mom. I would respect his perspective. But I WOULD still volunteer if that's what YOU want. But in some other way. Can you do the class newsletter? Library? Book fairs? volunteer in the class AHEAD of your son's so that you pave the way for his teacher next year to LOVE you!!!!!!!!
As far as how to approach it...... I would have a conversation with him. But he has to be calm. So, it may take a while with you stopping the conversation until he can be calm and talk to you. But have him tell you why he feels the way he feels. Tell him that in order for you to take his feelings into consideration he needs to be able to share with you what his feelings are and why he feels that way in a calm voice. Let him talk. Tell him you understand how he feels. Tell him you love him and that as a responsible mom you will be volunteering at his school and you thought instead of class mom you could do xyz. Don't ask his permission, but have him help you brainstorm with what other jobs you could have at his school - the ones he comes up with are the ones he's ok if you do.
And - if it makes you feel better..... my daughter was all about ms independent from 4 until 9. wouldn't cuddle on the couch to watch a movie. didn't want to sit next to me at dinner. didn't want me at school. Because it was HERS and she was establishing her own terf. But it was tough... there were days I had NO idea what she was doing at school (except for that I volunteered in the class above hers so I kept in contact with her teacher!!!)
But wow - she turned 10 and was all over me!!!!!! For the last year and 1/2 she sits next to me on the couch... wants me to watch movies with her... wants to talk about her friends and what she did that day and what book she read and how much she wants to marry Jacob (yeah.... I don't tell her he's a warewolf. cuz duh... she KNOWS).
I have to actively remember back to how sad I was when she wasn't so much about doing anything with me so I don't banish her to her room cuz there is only so much Twighlight I can take!!!!!
So, this too will pass. But out of it (hopefully) will be you guys figuring out what works for the both of you.
Good luck.