I was in an the same boat. Dd dropped a huge amount after birth and i struggled with bf, i didn't know what i was doing, she wouldn't latch and so we ended up bf and ff. She slept through the night at 7 weeks old for the first time and she only had 7 bf's a day and 3 x 2oz ff. I took her to get weighed regulary for the first 4 months and the centiles did go up and down so you just do what you need to do. Personally i would let him sleep, if he's hungry he'll ask for it. If you're worried about his weight just keep an eye on it.