You said it yourself, you've grown hugely, you're dominating SEO since you joined, and built loads of products. This is your 'partner' getting greedy after you've done a lot of hard work. It wouldn't have happened without you. Don't undersell yourself.
Your 40% is worth $400k based on that initial funding valuation, right? Assuming it's as successful as you seem to be implying, it is almost certainly worth more now.
Everyone is telling you to roll over but seriously, fuck them. This other toxic guy is the one who should be getting pushed out, not you.
The other investor ultimately has power in this situation, not you or him, so whoever convinces them that they are the person to go with gets the seat and gets to continue with the project.
If he has a good relationship there you're probably fucked, but results matter... and if you can prove you've done great stuff since joining and and have great plans for the future, he can be replaced.
To be clear: this guy has decided to blow up the project so he can get a bigger share, if it all fails now he can only blame himself.