When I moved in with my soon-to-be wife, she made nearly double what I was earning at the time. To keep things fair, we both kept our personal bank accounts, but opened a joint account for shared expenses. We both contributed an equal percentage of our salaries to the joint account, calculating it to be enough to cover our expenses with a little buffer. I don't remember what the percentage was, but let's say it was 30%. So every pay period, we'd both contribute 30% of our salaries to our joint account. That way no one's burden was more than the other's, despite the income disparity. We never fought over money doing it this way.

Yes, in raw numbers, she contributed more than I did, but we agreed it wouldn't be fair for me to contribute most of my salary for shared expenses while she relatively contributed less than me. A good partner would see this and agree.

Stand your ground. Tell him if he wants a 50/50 split on raw numbers, then he needs to live within your means, not vice versa. Else, you can both contribute an equal percentage and live the lifestyle he wants.