Ugh, that was so my #3 - it's like the realize they can get quality time in the middle of the night.
Day/night reversal does straighten out by 6 weeks, more or less, from everything I've read. The idea is to try to feed him every 3 hours during the day (which I know is super hard if he wants to sleep) and keep him out in the daylight and in the middle of everything during the day. And at night, do your best to stay low key. I used to go to bed pretty much after my older two and leave dh to deal with the newborn for a few hours -- then when he had to go to bed to get to work the next day, I'd take over and do the best I could. I was a definite zombie for a few weeks, but it did get better.