This happened a lot with my baby boy. And I finally figured out what the issue was after so much confusion. My boy’s pee stream was so fast and strong that it leaked out of the diaper before the diaper could even absorb it. This usually only happens to boy babies. One thing I did that helped a lot was facing his penis downwards when changing his diaper. Try it and let me know how that works out for you!
Is he sleeping on his sides? My boy (3m) also leaks top left/right when he turn to his side in his sleep