Why won't my 10 month old boy sleep?!?

Every night is pretty much the same. We have a hard time getting him to sleep and when, at last, he does (sometimes it will take a car ride) it's only for a couple of hours. The he wakes up, and depending of the time it is it's either for an hour or for the rest of the night. We've taken him to the doctor who made sure there was obvious problem. He doesn't seem to be teething right now, but when we give him some advil, he does sleep until the effects wear off. It makes me wonder if he's in some sort of pain but doctor didn't bother much! I can't really let him cry it out as he would wake his 22 months old sister (I figure one baby not sleeping is more then enough). It's been more then a month since he has had a good night. Worst nights, he barely sleeps and daddy and I have to help each other through the night (usually, daddy doesn't help much)

I'm getting exhausted.