Friendship etiquette

Hi, I'm visiting Japan. I've met lot of people and everyone is very helpful here. I've made a few friends, but I'm trouble by one of them. Where I come from it's pretty common to hang out casually between guy and girl (I'm a girl and he's a guy), and everyone split the expenses when we go out. But not a single time he would let me pay, arguing that I'm travelling and it's okay for him to pay. When I say "Next time, it's on me", he manages to pay behind my back. Is this common? I don't want to appear rude on insisting to pay but I feel bad and thus wanting to avoid seeing him.

Guys feel guilty when girls pay a half expenses. Guys have curious pride.
Let's say if you both visit a restaurant, he would pay for it, that is fine, then you ask me when you visit a cafe/fast food shop you pay for it, I think it works well. All guys remember how much he paid for girls. You pay a bit smaller amount than what he paid. It doesn't hurt his pride.

Oh I see, then next time, I'll try to pay for smaller amounts.
I guess, I'll never understand men's pride when it comes to this...
Thank you for the advice!

If you wish to keep this friendship purely platonic and on equal level (e.g., you are not treating him as your big brother), I suggest you insist to split all expenses.

Yes totally! I think I have been pretty clear where I stand, just platonic friendship. I live at the other side of the world. I'm leaving soon and not coming back in the near future and he knows it. Was it wrong that I let him pay previously? I don't want to send wrong/mixed signs. If I "sneak" pay next time, will it make it up to him?

He likes you.

I don't see this as a Japanese only thing. Across the globe, men pay womens way all the time. Hosts pay visitors/guests way all the time.

It was the same for me while i was in Japan, I'm a guy and while i was traveling in Japan i met another guy, we hang out and went out to eat he always offer to pay. When he introduce me to his family and his brother and girlfriend hang out with us, his brother offer to pay. At one point a had to aggressively offer to pay. i told him i wanted to pay to thank him for spending the time with me and showing me around.

To me its like that rule in Japan, you don't tip, but you don't offer to pay,

Well, I think it's not only Japan thing, all around the world guys pay women's bills. He really likes you)

not a single time he would let me pay, arguing that I'm travelling and it's okay for him to pay

It's not just Japan or a "guy vs gal" thing (or even "older vs younger" thing). I'm Japanese, and I get this all the time when I travel abroad from Japan. Even younger women tend to pay for everything for me. And when I'm not used to the country I'm not used to the payment system, so it's actually difficult to pay back when the local insists on paying. An easy way to "pay back" is to give gifts, especially something you've brought from your home country. Another way is to pay as you stand up for the ladies room or while he is away to the men's room.

What is personally hard to accept is that I told him about how it works back home. Usually, when I'm inviting friends I do not expect them to pay the next time because it makes me happy to treat them. However when they say, they want to pay, I let them do it since I know how guilty you can feel when you're invited all the time. That being said, paying a significant smaller amount seemed to work the best, although I still feel bad about being treated like I cannot sustain myself.