H.H.
It could be one of many or even a combination of things. If you've eliminated the previously mentioned possibilities, perhaps you need to think if anything has changed for him. Have you introduced any new foods? Something could be disagreeing with him; allergy, gas etc... Are you weening him from a bottle or pacifier? Has his day time routine changed at all; nap times; activity level. etc... Is he having bad dreams? My daughter started needing a night light at that age. She just didn't like the dark. (I have this awesome turtle night light that shines stars on the ceiling) These are all reasons I've run into. I know that if my daughter misses a nap or doesn't get enough exercise, she doesn't sleep well.
I also make sure to make bed time preparation as soothing as possible. I start about 25-45 minutes before actual bed time. I turn off the TV (it's too stimulating), I might give her a warm bath with a massage afterward. I always read a story, and then we turn out the light and sit and talk or cuddle for a few minutes. (On a side note I find that when she starts crying, I can distract her by asking her totally unrelated questions ex: "Listen. What was that noise? Did you like going to the beach? Wasn't it fun when...?) That way she feels she has had enough time with me, but when it's bed time, that's it; it's bed time. My formula for a while was 2 short stories, 2 songs, 2 bed. Now it's down to 1 story, the cuddle time is generally shorter and it's 1 - 2 songs.
It could just be a phase he's going through. For now, I suggest going back to survival mode. Sleep when he does. Otherwise, you won't have the patience to deal with him. Most importantly, follow your instincts with him. You know what you're doing (even it if doesn't always feel that way)
God Bless
H. H