I wouldn't rule out adjustment issues and sibling rivalry. However, as soon as I read 16 months, I thought *teething*. Where is he on his canine teeth? Have those come in yet? What about his molars? It may just now be starting... (Those were waaaay worse for my daughter than when the front teeth came in!) Perhaps his gums are hurting just enough that it wakes him up when he's between REM cycles or when he stirs in his sleep. This happened with my daughter when she was about the same age... I seem to recall 16-18 months was a whole new teething circus for us. And as soon as those teeth were cut,it was over; she slept better.
I am not an advocate of unnecessary medicating children for sleep. However I'm not going to let my child be miserable from teething. When she was going through it, I did give her infant motrin right before bed to help her sleep through the night (motrin not tylonal, because one weight appropriate dose should last 6-8 hours instead of 4 and you shouldn't have to medicate again). You could try giving a little motrin one night and see if he sleeps better, that might help confirm whether he's waking up due to teething pain or if its from new baby adjustment. Another sign of canines coming in is unbelievable drooling... My daughter wasn't a huge drooler after her front teeth came in but then for her side teeth, it hit overnight--BOOM--soaking wet shirts. (Of course, another sign to look for is the miserable cranky attitude that goes with it).
And if he is about to get those teeth in, then unfortunately you'll probably know it for sure in 3-5 days.