Increase his eating majorly throughout the day, all day long. It will take a few days for his body to register the change, but he'll sleep like a rock. Feed him healthy snacks and meals CONSTANTLY (not sugar of course and keep the juice early in the day-no fruit right before bed-give him some carbs before bed like a piece of bread before his last milk). Whenever you think of it, hand him something to eat, even if he seems content. If he's hungry he'll eat, if he's not, he won't. Always push. You may not think he's hungry, but his sleeping won't be so light if he's truly full.
I learned that trick form a mother of 12. My second ravenous boy would sleep through always, but whenever we were off schedule or a grandparent would skip the snacks he would start to wake really early in the morning or during the night. You have to be really diligent about it up until 2 years old.