This could just be a normal thing, some kids do just put a baby voice on sometimes for the annoying-the-hell-outta-parents value!

If he's not aware of doing it it may be a confidence thing, my daughter puts on a little squeaky voice when she wants to ask or tell me something that she thinks possibly won't go down so well, its probably best ignored, pointing out little habits often just makes them worse but if its really annoying you could say something like thats a really quiet voice your using, could you say it a little louder I can't really hear you. I'm sure its just a quirk that he'll grow out of and nothing to be concerned about.

Join the club. My 7 year old loves playing at babies and his friends at school join in. They all have baby names for each other.

I think it will be a phase like Thomas the Tank Engine.

I agree it is really annoying especially when his 3 yeart old sister joins in. I just don't answer when spoken to in the baby voice.