Callan or Clayton. When he is older he can choose whether he wants to use the full name or the nickname, with either of those. Both are somewhat unusual, at least around here. Both would sound mature when he's an adult. Jett sounds cute but will cute work on a business card when he's 35?
I always think it's good to go for names that work at five, at 35, and at 85. That doesn't have to mean extremely traditional names but it does mean -- in my opinion! -- avoiding names that sound trendy and adorable for children but might not sound mature for adults.
If you particularly like Jett, use it as a middle name.