I bet your son just has a different temperament and such that besets different sleeping patterns. My son (first and only right now) hit every regression hard and sleep has been elusive.
To be honest, I think it's the 9 month regression/progression. Language dev't along with the physical is really big around this time. Your routine is great for the most part. His nights span 10-11+ hrs a night which essentially makes his WU times not EWs, but with the time resettling at night, he is not sleeping an ideal amount. Was he consistently doing more than 11hrs a night before? NWs in the first 3-4 hrs after BT usually indicate OT. His broken night sleep, short naps in the morning, and time for resettling due to SA can be attributing to the early NW and EW.
Not much you can do wrt to OT except to help extend his morning nap if it's short. I would also aim to stick to the 9:00am nap in order to move the WU time. I also recommend trying to push 10-15 mins at a time to a later bedtime, possibly 6:30/7:00 to aim for a WU of ~6:00. I know this will be tough until we can get his naps consistently 1-1.5 hrs in length. If at all possible, stick to a minimum wake up time to expose DS to light... we usually say 6:00, but from 4:45'/5:15, maybe 5:45 to start. So even if he is wide awake, keep the lights off and amuse him in his cot or your bed until 5:30/5:45, then do a big wake up ritual- open blinds, sing morning song, etc...
Can we rule out teething? A growth spurt... is he eating more during the day?
I think your approach to his SA is spot on.. support him when necessary and I'm sure with a little support like GW, he'll go back to IS when this stage passes.
What do you think?