Hiya.
Right then. First off I would probably try a slightly earlier BT just to rule out OT from the teething - it might just help him to sleep a longer night & get you back on track. Of course, he may still wake at 4/4.30, but at least he will have had a wee bit more sleep IYSWIM?
WRT his routine, my first thoughts are that he's doing 4.75hrs A to BT, which at his age may be too short after a 2hr nap. So he sleeps a short night as a result, meaning he's got a 7.5-8hr A time to nap the next day, which is far too long. He needs the short A to BT b/c of the long day, but its not long enough to give a longer night so you get caught up in an UT/OT cycle IYSWIM.
So, you could try pushing his BT to 7pm, which may help nudge his WU a wee bit later. However, I would be wary of pushing much more b/c his day is already too long & this may compound the problems if he gets more OT.
Another option, given that he needs to be up at 6am, is a slightly earlier nap, say 11.30-1.30 & a 6.30pm BT. This gets you the extra A time to BT without you needing to stretch his day any longer, but obviously it depends on whether his nursery will flex his nap time at all.
Secondly, he's at the age where DT sleep needs can start to reduce. So you could try capping the nap shorter. Maybe start with 1h 45 so he sleeps 12-1.45pm. And then keep BT at 6.45. Then after a week cap the nap at 1.5hrs so its 12-1.30 with a 6.30pm BT - & see what happens with his WU. The key with nap capping is to make sure he doesn't go down OT at BT, so we don't want the A time to BT to increase - hence why it has come a little earlier. Hopefully his nights will extend to 11.5hrs & you get your 6am WU!
What do you think?