I think you pretty much have to let him be his baby self in private whenever he wants. Otherwise, he'll just do it in secret anyway. But I think you can make the deal with him that he goes to therapy for this too, and that you get to have meetings with the therapist too about him, from time to time. You mentioned cross-dressing, and it does seem to me that this baby acting phenomenon is similar to that in some ways. Your therapist says there isn't much research on this. But I bet there has been lots written by about this, especially by people who have this "fetish" (I'm not sure "fetish" is really the right term, but I bet some people would call it that). I bet you can find old books about it, and find tons about it on the Internet, somewhere. Will he ever outgrow it? Hard to say. Do some young people do cross-dressing for a while, then never do it again? I think so, in some cases. Perhaps he will simply learn to keep in control, and do it only on somewhat rare, special occasions. Is this behavior related to and caused by something in his past, like a trauma in his early childhood or babyhood? Maybe. Do you recall anything like that? Maybe he does. Is this behavior related to and caused by something in his present or recent past, such as being rejected by someone at school, or getting booted out of a clique at school, or puberty, or a car accident, or a fight at school, or being bullied at school, or a recent first sexual encounter that left him upset? Well, maybe his therapist could pull any such thing out of him. Good luck.