You guys really are similar to us! DH is a musician, though one of the biggest changes since before DS was born is that he now earns the majority of his money teaching rather than playing, whereas pre-DC it was the other way round. We're in London which also helps, but he doesn't do any touring or, tbh, many gigs at all these days. It was a conscious choice for him and was mainly driven by wanting to be around for the DC but also he really fell in love with teaching, so although he gets the odd twinge of jealousy seeing what his mates are up to, there's no resentment.
We do have my odd hours to content with, and the fact he works evenings teaching from home (as well as teaching in school 3 daytimes & all day Sat) but we are incredibly fortunate to have family locally to cover those odd times. DH does the childcare the other 2 daytimes.
I also work LTFT which reduces the number of long days/nights etc proportionally. I do "80%" at the moment, but on our Rota it works out as > 36 hrs pw average so I earn 90% of my FT equivalent salary.
Anyway none of that is much help to you! I would approach CMs/nurseries and explain the situation. You may be lucky and find some that are flexible, although I imagine they would all want you to commit to 1-2 days pw minimum. I read a post on here from a supply teacher who had a CM who she paid 1 day every week then others on top depending on her work (details might be wrong).
I would post on the Childcare board, you may well find others in the same situation. There is a reason many Drs have nannies though! (Cost effective from DC2 onwards I am told).