My son was an early speaker, obviously highly intelligent, and VERY articulate and clever. But the last several months he has reverted to baby talk. 90% of his speech is babbling and meaningless noises. It's driving me batty! Where is my son???? I CAN get him into a conversation but it is hard work on my part. It does seem like he is doing this on purpose. He has a half-sister at his dad's house that is one year old. OK, maybe he thinks she is cute and wants to talk on her level. Could he have Tourette's? Is this a normal phase? How can I get him back to his beautiful articulate self???
"Loss" of language in almost 5yo?
How puzzling! Has his speech vanished in all situations, or just at home with you or with his dad? What about when he plays with friends?
The fact that you CAN get him into a conversation if you work at it indicates that this is not likely to be a physical abnormality. And I'm not aware of any physical issue (including Tourette's) that can cause an articulate five year old to revert to babbling and meaningless noises. Of course, my training is in psychology, not in medicine, so you'll want to consult with his pediatrician.
But my guess is that this does have something to do with his half-sister, as you've suggested. He may well feel like she's getting an awful lot of attention and that if he babbles, too, he'll be just as adorable as she is. In fact, I would bet on it.
The question is, how can you get him to talk again? I think the best motivation will be his desire to communicate. If you don't understand him, he'll want to use words. (Although he may well be ingenious at making himself understood.) I'm not suggesting that you criticize his babbling, or even comment on it. Just that you act like you can't understand him, exactly as if he's a baby. But do this with a light touch. If you can get him giggling about this, it might clear up the whole thing. So when he babbles, be really silly in your responses and inability to understand ("Oh, is THIS what you want? You want to wear your shoe on your head? No? You want to drink your juice out of your shoe?!") Get him laughing and protesting. This will help him work through whatever anxieties are leading to this behavior.
I want to add that his behavior is a red flag that he has some need or feeling he's trying to address with this behavior. Many smart kids have younger siblings and don't resort to this. So whatever is going on is pretty strong. Does he feel he's lost his place in his Dad's heart, or home? Does he feel that his sister gets a real family while he doesn't? (Presuming that he doesn't have a stepdad.) I'm betting he doesn't know why he's doing this, meaning he didn't consciously decide to regress. He just wished he could be treated like her in some way, and began to act like this.
So while you want to help him giggle about this, having a rational conversation is a good idea too. You might want to introduce the conversation by asking him about the baby, and then observing that it can be really hard when there's a baby around because the baby needs a lot of attention and because he has to share his dad. "Sharing your parent can be really hard." You can ask if he thinks she's cute, and agree with him, but add that she is not nearly as cute as HE was as a baby. "You were SO cute....almost as cute as you are now! You have just gotten cuter every year...And now you can do so many things....ride a bike...play games....(insert here all the things he loves to do that a baby cannot do.) Too bad that babies can't do those things."
Finally, baby him. Right now, he isn't feeling terribly grownup. He needs a little babying. Don't worry, it won't last forever. But it might be just what he needs right now.
Good luck! And please let me know how this works out.