my little one is with me all day (except for maybe a few hours here and there when i am in class) and since I am fostering a litter of kittens right now (6 of the little buggers) my little one is getting tons of exercise. he has been sleeping through the night ever since my boyfriend finally allowed him to sleep w/us.
maybe your little one misses you at night
also my babe doesn't go to bed until 1ish (when I finally get to bed) and will sleep until noon if i let him... the college girls best friend
add a bit more exercise to his routine and it should work out