Only 2 hours at a time at 5 months???

My little guy is 5 months old and will not sleep for more than 2 hours at a time. We do a nightly routine of nursing, eating rice cereal, putting on jammies, and then rocking and nursing. Sometimes he is really tired and falls asleep right away, sometimes he doesn't want to sit still and we have to wait and try again later. 
He will sleep good for about an hour or two then be back up. Sometimes I can soothe him back to sleep or let him soothe himself, but sometimes he won't calm down unless I pick him up and nurse him. Then I put him back down and repeat it all an hour or two later. Usually I end up getting exhausted and bringing him in bed sometime after midnight so I can wake up for less time when he wakes up. Even sleeping next to me he won't sleep more than two hours. 
We have tried varying his nap lengths and times at daycare, letting him sleep on his tummy, sleeping in a swing, giving him a bottle before bed in case he isn't getting enough from me, letting him sleep in another room, swaddling, swaddling just his arms, just one arm, and just his tummy/chest, changing the temperature, different bassinets and the crib, and stuffed animals. He will not take a pacifier to save his life. 
I am getting desperate for sleep. Any ideas? Anyone have similar issues?? He's such a happy baby during the day but night is a disaster. He has been like this since birth!