Sleeping "through" the night question

Brittney • I`m 32 years old and have a almost 2 year old son (bday 9/3) and pregnant with our little girl (due 10/4)
Since breastmilk digests quicker, what will sleeping through the the night look like for a breastfed baby? My LO is almost 6 weeks old and he goes down between 9-10:30pm then sleeps for 3.5-4.5 hours but then is up every 1.5-3 hours after that until we get out of bed (usually around 8am). I feel pretty good about this as it is SOO much better then it was a few weeks ago but at what point do you think I can expect that second stretch to last longer? He won't empty both breasts at in the middle of the night and I think that's the main issue. He is just too tired. Also around 4:30am his is WIDE awake. I have to have him sleep on my chest since he won't go back down in his bassinet. But that works for us. I love to cuddle!