R.R.
Anything new going on in his life? Have naps changed? Keep TV watching to pre-dinner and make sure his bedtime routine includes time to relax; a warm, quiet bath, no roughhousing, a rubdown with night time lotion is nice, and a story or two.
Being overtired can also cause this so if he's not getting enough sleep you may want to tweak his bedtime and up it by 30 minutes. Also a light snack with some carbs or calcium before bed may help him to sleep through the night; like a slice of whole wheat toast with a bit of peanut butter on it, a small cup of cottage cheese with some banana diced on top, a cup of milk or yogurt.
Hope he goes back to sleeping all night soon!