He won't want to hear this but I would split the equity based on the work you each do or have done, not value the idea at all.
Let me explain. There are 2 types of ideas. One where the expression of the idea is all you need. Things like "I have an idea of a facebook for cat lovers who also enjoy beer." Those ideas are most prevalent and are worth nothing. If you had that idea, so did 15,000 other beer/cat lovers. It's the execution of these ideas that brings value - if you can actually make the thing and (more importantly) get traction. Remember, Zuckerberg's idea was not novel. Harvard U did it on paper, myspace and others did it on the internet. But he executed so well and that's what made it valuable. So split the equity based on the work you each do to execute.
The second kind of idea is much more rare. That's an idea of the form "I have an idea how to launch payloads into orbit for 1/2 the cost of current technologies." This kind of idea needs a lot more than just the expression of it. In fact, already a whole lot of work has gone into it before you can even suggest it. If your partner's idea is of this type then indeed give him equity based on the work he's done to get the thing as far as it is and give each of you additional equity for the work you do going forward.
2 other points (though you didn't ask) 1) look up founder vesting and put a vesting schedule into place. 2) None of this including the prototype or patent will be worth much if no one wants to buy it. So put some effort into CUSTOMER DEVELOPMENT before you spend a lot of money or time on building the product.