Though I also prefer puppy over boy, this reminds me of the discussion that crops up every now and then about doing away with boyfriend/girlfriend and replacing them with another word like datemate; it is a bit catchy.
But I have given it way too much thought and have come to the conclusion that when using boy or girl in romantic situations or in friendships, what you're really saying is the person makes you feel young, happy, and carefree, harkening back to how you felt in a time where you had less responsibilities as well. Springtime, in more poetic terms. This couple does fall into this season of love but because of all the hanky panky, I view it more as a summer glow, you know?
I'll always prefer officials over other translations(specially mtl) but the officials have his term of endearment as "Boy" and the fan translation has it as "Puppy". I def prefer puppy over boy. Like I can kind of get it in the terms of dog ownership calling ur dog "boy" lovingly but it's super weird in an actual relationship