It's true, he's very immature. I keep encouraging him to grow out of his childish behavior and act like a grown up if he wants to be paired next season.
It was a defensive bite for sure - he's terrified of being handled and will almost always strike if he can see me. If he's in his hide and I lift it up and scoop him, no problem - but he was curled up next to it and had a clear line of sight. Calms down once picked up and gets all "who, me?" about it.