Honestly hun, if A times are age appropriate and he's an independent sleeper I'm not sure there is anything we can "do" or "fix" ya know? If you set the scene with wind down, use proper A times, independent sleep, and naps are still short I think it is just a matter of him developmentally learning how to sleep longer. I absolutely found that was the case with both mine anyway, no amount of analyzing and tweaking was going to solve it before they were developmentally ready to do it.