I posted about this recently so I'm mostly paraphrasing myself ;-) - my DS (17 mos) wakes in the middle of the night and stays awake for 2-3 hours before settling back down - he doesn't really complain or cry, just seems to be trying really hard to get back to sleep by singing, rocking, playing the music thing that's in his crib, etc. and to be unable to do so. It's totally disruptive for both him and me (DD sleeps like a log thank goodness). He's been very good at putting himself to sleep since I ferberized him at 6.5 months, so it's not that.
I did a bunch of research and in particular read the Ferber chapter on sleep issues and determined that what it probably is, is what Ferber calls "in the crib/bed too long" i.e., I'm expecting too much sleep from him. Ferber says it's common for parents to think babies need more sleep than they really do (for the 12-18 mo range the average sleep requirement is 11-12 hours total including naps so guessing for 19 mos it's a smidge less) and it can cause either struggles at bedtime, or too early wakings, or middle of the night wakings. DS would be happy to sleep 3 hours in the afternoon, but that usually tips his total sleep to well over 12 hours, and that's when he starts up with the night wakings. By my calculation your ds is "in the crib" around 13.5 hours, so it may be that same problem, esp if he may also be a kid who needs less than what is typical anyway. Try an earlier wake up, later bedtime, or shorter naptime and see what happens. For us it works best to adjust the naptime or the wakeup time since I can't bear to have a later bedtime for my kids, LOL. GL!
Lucky single mama to DD 5/08 and DS 6/11