My sympathies, LDRs are not easy. They can work but many don’t. Like any and all relationships, they need communication.
From what you have said, I wonder if you are feeling essentially isolated from your partner. You don’t feel that he’s putting any effort in and you are - this is a natural consequence - drifting away from him. When you try to raise the issue he tells you it’s your problem to solve your feelings, not his issue to put more into the relationship. For what it is worth, based on that snippet, it looks more like a him issue than a you issue.
I had a 4 year LDR. 1 year was transatlantic and 3 years were cross-Europe. And then we moved in together. We always knew that LDR couldn’t be permanent and, fortunately for us, at the point where the LDR would have gone back to having an ocean in the middle of it we were able to move together.
All along the way, we had much, much more contact than you seem to be able to extract from “the love of your life.” He wanted to be in contact daily. Even if just for 5 minutes. And this from someone who is hopeless at expressing emotion.
Your partner seems to think that the relationship can be popped into the salad drawer and will do just fine until he takes it out again when he sees you in October. But it won’t. If you were happy with the degree of engagement you were receiving from him and your feelings were not being alienated it would be fine, but you are not fine and you both need to put more into this if you are going to survive this.
A previous poster asked if your time apart has an end date or if it is open ended? That’s an important point. If you are away on a contract or an academic assignment for a year or two then you can navigate this. If Japan-US (with a 12-14 hour time zone difference), is permanent, then you may need to reconsider.
A lot of me wants to say, see how you both feel when you are together and can interact normally. And if you both want to call it quits because feelings have moved on use your time to have a great Japanese holiday as friends. But that’s a bit over optimistic. I think you need better communication before October.
Don’t waste an amazing opportunity of being in an incredible country like Japan because you have a partner who won’t partner you in a relationship and wants to leave it all to you. Give him a wake up call. If he won’t wake up, go and enjoy Japan instead.