Hi L.-
With my 2, before they were 4 months old, we kept them up until 11, when we went to bed. We let them have 3 naps a day- 9 or 10am, 2-3 pm, and 5 or 6- but they only napped for about 1hr to 45minutes.
Then when we put them down we give them one last bottle and change, put them down already asleep (I know many books say to put them down slightly awake, but at this age I think it is too young) and they would sleep until about 4, and wake for a bottle and go right back out. I also would suggest not changing the diaper, unless it is leaking or poopy, that way you won't wake him up more.
It really sounds like he has his day and nights confused. I haven't had to deal with that myself, but I bet if you keep the light level up and take him out in the stroller he might try to stay up. Just try your best to keep him awake more during the day. It can be rough- I remember trying to keep my first up until 11 was tough, but well worth it.
Both my kids were sleeping from 11 to 6 by 4 months, then we bumped up their bedtime to 8, and woke them for a bottle at 11.
By 6 months they were sleeping 8 to 6! (well 5, in the summer- uck!)
Good luck- it will work itself out over time, too- don't worry!