[stylist] the best way to fix this sentence

[Vejas]

Hi all,
I have a new e-mail account for my list-servs, as you can 
probably tell (yes, I ski too.)
I have a question about the best way to phrase the following 
sentence:
"In my opinion, he is such a child."
The adult in my story is someone who, despite being mature, still 
appreciates things from when he was younger, and the narrator is 
a self-centered teen.  Is it better to use such a child, like a 
child, or just stick to a child?
Thanks so much.
Vejas