I also think college. I know they're going to different colleges in different countries, but maybe "absent makes the heart grow fonder" works for them!
Hufflebecks: plus they make new friends and new best friends and new best best friends but for some reason none of them can compare to each other? then they're like huh
Drew’s rambling to his college roommate about Satoru, roommate says something like “wow, you really must miss your boyfriend huh,” Drew.exe stops working, cue realization
Satoru talking to Drew on the phone and one of his friends tries to interrupt but someone else goes "Hey leave him alone he's talking to his boyfriend back in America!" in Japanese and Satoru chokes and Drew's like "what did they say?" and Satoru's like "something about school haha" but Drew hears them teasing him in japanese so he's just like ???
Would be so amazed that his son's boyf is as brilliant as him, but of course he also makes his boy happy in their nerdy land so that's more important. XD