Working part time isn't him doing you a favour.
It's him using that as justification for putting all the housework, childcare, "default parent" status, carrying of the mental load involved in parenting and running a home, onto you. Even if he worked 60hrs a week, you'd still work more because you'd be effectively "on call" 24/7 doing everything and running round after him as well as your son.
Being part time creates the additional power imbalance of him earning more even if you had the same hourly rate. In a selfish person that equates to them getting to spend the money on things they value and you having to justify things you want or need, to him.
Being part time reduces your earnings, making it harder for you to leave him should you decide you want to at some point.
In short, it's a way of trapping you. Of making you feel that it's easier to do what he wants than to separate. Or in other words, your decisions are no longer your own, due to the potential negative consequences of standing up for what you truly want or need.
It's a way of him effectively having greater control over you. Even if he's not consciously setting out to do that and I wouldn't be so sure it's unconscious that's the end result. As he's a selfish person who doesn't have your best interests at heart, this is a dangerous thing.
Interesting that he started on with how unhappy he was and how you all need to go live in Australia isolated from your support network a year ago and your son is a year old. Coincidence? Or an answer to the question, if he's going to Australia regardless then why have a child with you? A child makes you more likely to try to make the marriage work, even if you're the only one trying! A child makes you less likely to leave him once you're in Australia, because you'd lose the child if you came home and if you're financially reliant on him how do you leave him and afford to stay in Australia?
His plan to move yearly whilst still in the UK effectively prevents you from putting down roots anywhere too. And possibly makes a move to Australia, with the promise genuine or fake of stability, look more appealing. It's possible he's had his life and your place in it mapped out for a very long time.
Re: worried you're saying no out of principal. What principal? Believing you're worth more than his selfish ultimatum? Believing that a marriage should be equal and he should consider your views before making a decision, instead of expecting you to do whatever he wants all the time? Believing that this wouldn't be a good life choice for you? Because there's nothing whatsoever wrong with any of those principles. If your gut instinct says no - listen to it.
Of course the unpaid internships paid off for him, you were funding everything! He chose to do something for himself at your expense.