Welcome to the world of pre-teens' grandmas, writergrandma! My 12.1/2 year old grandson is exactly the same. He's turned into Kevin the Teenager, who only talks at length about things I don't have a clue about (Minecraft, Bridge Constructor, geeky computer website construction, owning a server?!?!), which enables him to patronise me. Otherwise, he responds in one syllable utterances, and avoids eye contact. I know he loves me, but for now he doesn't want me hanging round him - it makes him look uncool. When I pick him up from school occasionally, he walks past my car first, then comes back and gets in. Doesn't want his school pals seeing him getting a lift from his gran!
Girls mature earlier than boys, and I remember my twin daughters being like this at that age. They were polite when they were with adults, but any close family member aged over 30 was seen as a wrinkly old person who they felt they could ignore. We would have words every now and again, and they grew out of it in due course.