At the time I did it with my DS naps were regular so it was only the night sleep which was in question, yours does have the added 'complication' of sometimes getting a bit of nap time in on some days and not others, I think I'd pretend like he gets none, offer none, not encourage any (ie don't purposely try to catch him up on his sleep by taking him for a car ride to get him to sleep). I don't know how feasible it would be to avoid car journeys all together, if you have to go out then you have to go out but if he does nod off I would wake him the second you arrive somewhere rather than let him sleep.
Really your guess is as good as mine but if you are happy to go with guesses...
I'd aim initially for an 11hr night (with a longer term hope for longer maybe 11.5hr or 12hr but that's no naps at all) which I'm guessing at based on him doing 10.5hr night after a 40 min nap *and* managing to sleep past 6/6.30), so working backwards with WU at 7am that would be something like an 8pm BT where this might end up (I actually think you could end up with 7.30pm BT if he moves his Wu and if the naps are zero but I am hesitant to give you too much hope yk?). Be prepared to go beyond the 'planned BT' this to force the later wake up by over-riding his body clock
Do bare in mind nights are entirely likely to be shorter than the 11hr we'd hope for whilst the move is made because initially his body clock will continue to wake him at 6am, only after he shifts his Wu can you hope for nights to extend to a better time.
As he sometimes does 7.30pm BTs I'd probably start there:
night 1 7.30pm (expect 6am WU) 10.5hr night
night 2 8.00pm (expect 6am WU) 10hr night
night 3 8.30pm (expect 6am WU but might shift at this point, might also be getting OT)
night 4 9.00pm (FX for later WU now)
night 5 9.00pm (holding at this time to hope for the later WU or to ensure the body clock doesn't revert back to 6am)
At the point morning WU moves later, and reaches 7am, I'd hold at that BT whatever it is for a day or 2 then night 3 bring it 15 mins earlier, again next night etc until you reach 8am.
Make sense? Please bare in mind I'm only guessing.
If the whole thing is a total wash out you will likely have 1 week of hell followed by maybe another week of getting back on track with EBTs. The idea of this is not that it goes on for ages, it is done pretty fast so that OT comes quickly.