>>6 That there is good advice.
Hiroki, it's lovely to hear how much you care about him. Explain your worries to him - communication is pretty much the best thing in this kind of situation.
Tell him how important this is to you, and I'm sure he'll understand. But, like Arya said, don't make it sound like it's an end-of-the-world kind of deal. Just do your best to work together and achieve that goal.
I wish you both the best of luck, and sorry this probably isn't super helpful but I'm sure you'll be fine :)