I know how you feel! Since your lo knows how to fall asleep and is not teething, not ill, not really crying in the middle of the night, the only possibility I can think of is...could he be ready to stay awake a little longer between his naps?
DD went through this stage along with the early wakings and short naps at around 8 mos too. After ruling out all other possibilities, I decided to increase her wake time by 15 mins between naps and bedtime until the situation was resolve.
What is your day like? Try to follow his cues for the next few days to see when he is really ready to sleep. HTH
PS I guess that being a detective is also part of the job description! :wink: