Honestly I would say that his turn should not have ended, so let him land the planes where he wants. If it ends up making a huge difference to Italy, then re-do Italy’s turn.
I can’t imagine playing with someone who made me lose the planes in that situation. I would quit the game right then. i don’t care if it’s a league game, someone who does that is not someone I would ever want to play with.
In our games, when this happens, we let the player land them wherever they want, even if it’s a whole round later. It’s really not a big deal.
Even with convoy raiding, if we forget it until the next round, just roll it then. It shouldn’t make a difference if they lose the money at the end of their last turn or the beginning of their next turn.