K.L.
You might be making it too interesting staying awake. Tea has caffeine, and food gives him the energy and then his tummy is busy digesting it. Maybe it's something he is getting for dinner that keeps him up. You have probably heard about some people who have extreme dreams after eating spicy foods. Some foods are like a time release capsule of energy and he may be one who is sensitive to that. Or maybe his naptime was too late or too long but you just didn't notice, and now he knows you will entertain him late nights. I guess you could just put him to bed and say that's it. If he has no audience and nothing else to do maybe he will just go to sleep. Be glad it's only now and then, and not every night.