Your pitch is too broad. What does he want from the language? This field is vast, so you're unlikely to get what you want unless you say what you want. There is no risk of someone "stealing" your ideas because PL is not a competitive field. The number of people who could run away with your ideas are vanishingly few, and those people are already working on their own projects, which will have an entirely different philosophy and design requirements from whatever you're doing. At most there might be a little bit of cross-pollination, but what do you care? As you say, you're going to open source the language.

Right now the only thing interesting about you is money, and anyone who would be qualified to work on this project won't have any issue finding employment. Additionally you're talking about the project's funding, not the salary, so even that isn't very interesting. Given how poorly most projects are managed $10 million is barely anything. Your primary hook should be the project itself, and everything else would just be cherries on top.

Is this remote work? Will we be expected to move? Where are you located? Don't make us do research into whatever you are when you're the one who's desperate for a rare talent.

tl;dr: be more up front, focus on what's interesting, and cut the fat out of your pitch