How do I get my 5 month old to quit waking up 5 or 6 times a night?

I have a 5 month old son. He goes to sleep about 9:30 or 10 pm. He goes to sleep with a minimal fuss but wakes up at about 11 and continues to wake up every 30 mins or so until about 2:30. He then sleeps until about 5:00 and then again at 6am. When he wakes up I put his pacifier back in his mouth and I don't let him cry for very long because he is in the same room as my 20 month old. i have started giving him a little cereal at night but it doesn't seem to help! Any advice??!!!