My ds was much like this at that age! Fast asleep all day, restless at night! From all the reading that I did, it seems that babies do naturally outgrow day/night reversal by 6 weeks -- typically you're supposed to keep doing what you're doing, wake them every few hours to eat and keep them out in so I where where there is sunlight during the day. Keep things low key at night -- we worked on estaishing a BT routine early on as well. Honestly I think time did the trick more than anything else -- that being said, lots of (((( hugs)))) in be meantime, it's really hard to have to be awake during the day for your big one and all night for your little one!
I'd also check to make sure you're burping well and that you're not dealing with any obvious discomfort. But if he's fine and sleeping well during the day, it does sound a lot like day/night reversal.