Baby waking up out of habit or necessity?

My 5 month old has been sleeping through the night for several months. Every once in a while he would wake up in the middle of the night, but a bottle would put him back to sleep. For the most part, he sleeps 8-9 hours straight. For the past week (coincidentally the day we started rice cereal) he is waking up around 2am every night, I give him a bottle, he finishes it, pretty much sleeping while he is eating it. He doesn't take a pacifier and if I try not to give him the bottle he cries. I want to know how can I figure out if he is having to wake up because he is actually hungry or its just become a habit because its aroudn the same time every night. He won't take more than 4.5 ounces of milk/formula at a time and eats about every 2-3 hours still. About 30 oz a day I think. I started giving him rice cereal in the early evening. Your thoughts?