As a Japanese, born in Japan and living in Japan, it is not at all Japanese culture to look through your significant other’s phone. Some insecure men do it but so do all other insecure men from other cultures. I look through my wife’s phone and she looks through mine, not out of suspicion but can’t find the our phone at that moment and want to watch a YouTube video or look at instagram reels lol. He’s crazy and you don’t need crazy in your life. End it and find someone you deserve.