Can't reply to everyone now, but just saw Ju (ace names!) & wanted to say that you shouldn't let her coerce you into giving formula if you don't want to.
If you want to try expressing, do it from one side whilst he's feeding at the other to get maximum milk.
Remember skin to skin - spend as much time as possible cuddled up in bed with him in just his nappy & you topless.
When he's finished feeding on one side, offer him the other - & then the first side again when he's finished there.
Don't let him sleep for too long before feeds. Wake him every 2 hours during the day & every 4 at night if he's not waking up. If he falls asleep very quickly during a feed, or is reluctant to wake up, tickle his feet or under his chin.
You probably know all of those things, but thought they could do with restating. & don't forget to take care of yourself, too - keep a bottle of water with you all the time in case you get thirsty.
Don't let her talk you into FF unless you're happy with that - it will quite possibly damage your supply, which won't help.
He's a really good size though - don't worry yourself too much. Some babies do just lose a little extra weight. What's his poo like? How many changes are you doing a day? xx