Barbara,
I've written in before, and have actually read your book (which I love), but it doesn't address this particular issue...

Dear Michelle,

For reasons you may never know, he's feeling stressed. That he only does this with you is the tip-off. You're the person with whom he feels safest. That he chooses baby-talk instead of some other behavior tells me that he's looking for comfort. My guess is, he's feeling ambivalent, maybe unsure of himself, about demands that are being placed on him academically, socially, physically, emotionally.

I think this is within a normal range of development. I like to think of each stage of development as being divided into thirds: One-third of the time, a child's behavior is exactly where you'd expect it to be, developmentally. One-third of the time -- when he's feeling powerful and strong and all the stars are aligned -- he feels as if he could conquer the world. That's when he gets ahead of himself, developmentally. And one-third of the time, when he's tired or he's put out a lot of emotional or physical effort, he feels needy and incapable. That's when he regresses.

The baby-talk is obviously the regressive one-third. Here's the problem: When you ignore it, he's not getting the comfort he needs so he keeps doing it because he doesn't know any other way to get what he needs. So I'd try the opposite approach.

In the sing-songy voice you'd use with a baby, tell him, "Oh, my little baby, baby! Do you need a cuddle? Do you need a mommy hug?" (Do this only if no one else is around. In fact, I'm assuming he only does this when there's no one else there.*) Then, rock him or cuddle or whatever -- this should be quick, no more than a couple minutes -- and then, in your regular voice but while you're still in the comfort-giving position, grant his wish in fantasy: "Boy, wouldn't it be nice to be a baby all over again? And not have any big boy responsibilities? Sometimes, I'd like that for me, too." And then, poof, the spell is broken.

Once you've indulged him, I'm guessing he'll go back to regular voice and, after time, need to do this less and less. In fact, after this has happened a few times, offer him a short-hand: "Next time you want to cuddle, just ask me for an MH. Mommy Hug. It'll be our code."

*If he does baby-talk with you in front of other people, or if this happens many times a week, I'd consider it a red flag and seek professional advice.

Thanks for the shout-out, Michelle, about the book! which is now available in e-book!