You could steal the 'Uneducated' trait Patch used to have? If he was trained to do only one thing from a young age he wouldn't really have a wide knowledge base. On the other hand you could give him something like 'Bookworm', since he's more eager to learn things now since he missed the opportunity to do so when he was young, and so spends a lot of time reading anything he can.

Or just keep it simple and give him either 'Prankster' or 'Immature' as a trait, both of which would having him acting a little childishly, even during missions.

Or follow Patch's example and go for an obvious character flaw, like 'Substance Abuse', 'Gambling Addict' or 'Adrenaline Junkie'?