Is he already in it? How does he behave? I have my tiniest baby in a 20L- which seems huge for her, but she is all over it all the time, obviously very comfortable. She has vines on all the walls that she uses for traveling, but often she'll just lay out in plain sight at the front. She's a bloodred- I guess she thinks she's tough! (I'd rather have put her in a ten- but had an empty 20L).