I'd recommend looking at your school's or district's math curriculum for K, 1st, and 2nd grades. Our K math curriculum (which follows the Common Core State Standards) does not start adding single digit numbers until after winter break, and they focus on adding to no more than 10 by the end of the K school year. 1st semester K "math" is number and shape recognition, counting, and patterns.
We gave our K teacher a list of DS's math skills two years ago when he started K, compared them to the district's curriculum, which clearly showed he was working at a 2nd grade level, and got him tested and accelerated by a year in math. Our K teacher said she was still working with some of her kids on just learning to count to 10, and she would not be able to provide him the challenge he needed in class.
Our DD5 is now in K, already doing 1st grade math books at home, but we aren't pushing it with her because she's not as far along as DS was at that age, and the administration has changed and is no longer friendly to subject-level acceleration. (We are fighting to maintain it for DS7 this year.) If you look at the curricula, get a good feel for where your DS's skills are in relation to the different grades, that may help you decide what you want to ask for.
Regarding expectations, if you're in a regular program (i.e., non-GT) and your child is scoring 99%ile in reading and 91%ile in math, you are likely going to have to do a lot of advocacy to get your DS the challenge he needs. If your child has been counting to 10 since they were 2yo, it can be hard to understand that some kids really can't count to 10 when they start K, but there are a lot who can't. If you have the time, volunteering an hour in the classroom during reading or math will also help you get a feel for the other students' abilities and how far of an outlier your DS is. If there are other kids with similar abilities, it will be easier for the teacher to differentiate within the classroom if she's willing. If he's the only one and the rest of the kids are fairly far below his abilities, it gets harder for the teacher to accommodate his needs as well.
