It sounds like he may be overtired and having difficulty settling into a deep, restful sleep. Has he ever napped for longer than an hour? If yes, we can try to get him napping longer. If not, we can find other solutions. Often fixing the day sleep can improve night sleep. If he won't give you longer naps, you could try adding in a third nap of 30-45 minutes around 5 or bring bedtime quite a bit earlier (possibly 6).
How does he wake from naps? What is his mood like during the day? What do you do to settle him when he wakes at night?