You could try over accentuating day and night. At night make sure everything id super dark and quiet. I agree with the bedtime routine as well.
And during the day, i dont know, try a strict routine with daft things like bottle, play, nap, bottle play nap then at night when he gets his bottle, read him a story in a dark room.
I know it sounds daft but after a few days he should start to get it, being able to tell the difference.
Just a thought