[Pogopuschel]

Guys are all the same, Japanese or not. What your friend says may have been true several generations ago (not even then I would guess). To me it sounds like he just wants to have fun. The fact that he is Japanese doesn't change anything, he could as well have been American/European/Asian and your situation would be the same. Just treat him like a guy from your home country. Japanese guys are not anymore conservative, trust me. They may be shy, but not conservative.

M: "Let's do it"
F: "Noooo, not before we're boyfriend and girlfriend!!"
M: "Why??"
F: "(Insert long text)"
M: "Ahh...ohh... okay, let's be bf/gf then"
M/F: "Alright, let's do it"