Anybody who has been through the correct process to receive British Citizenship is legally British. You may not think it's a good thing, but it's a fact - so to say he is not British is just plain wrong. In your opinion he may not be - but I'm afraid your opinion does not count for anything legally.
Anyway this thread is about Kyle Edmund. Back on topic. He is a good prospect - not Murray standard of success, but I think he should have a good career.