How old is your son? I have a feeling our sons might be around the same age. Mine is turning two next week!
I’ve wondered about the “pretending not to understand Japanese” approach – like yours, I don’t think mine understands that there are two languages that he speaks right now, so this isn’t something I would say to him now, but I’ve wondered about it when he gets older. He will learn that it’s not true eventually, but apparently if you stick to that, they believe in it for quite some time! (Even if they see you speaking Japanese, it doesn’t seem to click right away, haha)