Hi,

He sounds like a delightful, settled little baby. He's perhaps a little "too good" if you know what I mean. I seem to hear this commonly with 3rd or 4th babies. You will find that, with a few simple, short-term measures you will be able to turn things around. Suggesting you put your baby on formula is completely unhelpful.
What I would suggest that you do is to feed your baby every 3 hours. Make sure he has both sides - ie no more than 20 mins on one side, and then *wake* him a bit (burping, nappy change etc) and give him the other side. Ensure he has at least 8 feeds in 24 hours. You will find he turns around quickly. He will probably become much more demanding. For a few days he may seem *ravenous* and you will wonder where your placid little baby went - remember this is a really good sign! You will soon be able to "take off the training wheels" and let him call the shots again - wehn his weightgain improves.