We went through the same thing with our now 26 month old son. (And, my older son was like yours - always slept through the night.) My pediatrician recommended that we let him cry until he fell back asleep. It was the hardest thing to do! In fact, I had to go outside the first night because he cried for so long. (For some reason, too, it was better if my husband went in the room and just patted him and let him know it was still "night night time" without picking him up at all, if his crying just became too hysterical.) Within the week, though, he was able to comfort himself back to sleep. It was a real blessing when he started sleeping better without our assistance because we were finally able to get some uninterrupted sleep!