OP - you really are resistant to any of the part time, school hours jobs suggestions. So if you really want to find a way to 'sell' this to your DH, i'd go to him with this idea:
Firstly: you give your DS the first term (so until Christmas) with you at home, lots of DCs get very ill in the first term and it might be a lot of change to go from being at home with you to school full days and then wrap around childcare.
In this time, you will focus on writing for 4 of the 5.5 hours a day your DS will be out of the house (allowing for you to get to/from school for drop off/pick up), the rest will be housework and personal time.
You will also look at ways to start getting things moving with your book - by October half term, you will have finished all edits to your first chapter and synopsis and have sent to several agents to see if you can get representation.
You will also look at writing other paid for articles to bring money into the family.
At Christmas, you will look at where you are up to, if from your articles you are able to bring in more than £500 a month, you'll ask to carry on with that.
If not, then you'll look for part time jobs, ideally 2-3 days a week so still allowing some time to do some writing, or school hours, but on that you will expect him to understand you will then have no free time (as your non-working time will be looking after DS) so you want 1 day of the weekends to write.
However, personally, I do think you should look for a part time job now, even if it's a 'filling time' job, something to show willing to be part of the team. If your writing gets to the stage it brings in some money, great, you can quit. But if you can't bring money in doing it, it's time to realise it's just a hobby, and it is as selfish to say you want to focus on that as if you'd said you'd like to play golf every day.