Hey Kirsty,
It took me a while to realize that the 12 hour day/night isn't an exact science and that bedtime has less to do with what time they wake, and more to do with how much A time they handle. So if baby wakes at 6am, that doesn't mean baby has to be in bed at 6pm. Instead, it's how much A time they can handle after that last afternoon nap that dictates the bedtime. Once I started following that philosophy, I relaxed a bunch and the sleeps went well. For us that means that bedtime lands a little differently every day...
Believe it or not, an earlier bedtime does not equal earlier wake time a lot of the time. You might just find that if he wakes from his nap at 3pm, and you put him to bed at 6pm, he may sleep through to 6am.
Although I guess some babies are just early risers, so I don't know what you can do to make him sleep later?!?!?
One last though - at 9 months, most babies are doing 3 hours-3.5 hours A time, so maybe he's ready for you to extend that A time a bit? Maybe by 10-15 minutes and see how it goes? For us, morning A time is always short, like 2-2.5 hours, but the rest of the day the A times are 3.5-4 hours.
Jana