My son is around 11 months and he started doing the same thing around ten months. At first I was concerned, but then I realized he just learned a new trick. Now, WHen I shake my head no, he does it back

I'm not sure that he knows that it means "no," but he knows it means something and he's excited to communicate with me
I think it's normal If you want to be safe, I say go get him checked out, but my son is about your age, and does the same thing