Some kids that young are just not on the day/night clock yet:
Most infants take about 12 weeks to show day-night rhythms in the production of melatonin, the “sleep hormone” (Rivkees 2003). Circadian changes in cortisol--a hormone that helps regulate waking--may take even longer to emerge (Rivkees 2003). And, overall, babies may take 3-5 months before they “settle” at night--meaning that they sleep for more than 5 hours at a stretch (Jenni et al 2006; Pinilla and Birch 1993). - See more at: http://www.parentingscience.com/newborn-sleep.html#sthash.dJ3oezEQ.dpuf
You can try a few things to get your kid on the right schedule though:
- Make sure he gets exposure to natural light
- Minimize stimulation when they do wake up at night (don't talk or sing or turn on the lights, just feed or diaper and go back to bed)
- Play hard during the day - make sure your kiddo gets lots of stimulation and activity during the day
- Try to limit daytime sleep to about 2.5 hours at a stretch
For that last one, I'm not saying wake him up after 2.5 hours, but you could open the door to his room or raise the shades and let him wake up. You definitely need to help him see the distinction between "dark sleeping time" and "bright waking time" even at naptime.