Is he walking yet? It's right around the time he could be starting to work on that and new skills screw up sleep IME and there isn't much to do beyond ride it out. You can 'practice' getting down in the crib and see if it helps or if all else fails make the room 100% baby proof and put the mattress on the floor and let him sleep there.
We rarely go back in his room when he stands up. We let him figure it out himself and eventually he settles back down. He loves when we come back into his room and attend to him and often thinks that nap time is over if we come back in, so we keep it to a minimum.
With V at that age (or maybe a month or two earlier) we were still helping him fall asleep for every nap and bedtime. If we didn't he'd go to the edge of the crib and pull to standing. Then we just let him get tired one night and ... didn't go in. He flopped down and eventually got the picture.
Almost every transitional stage has been terrible for sleep for O. He knows what he wants to do, can't quite do it, and must think about it all night and let it drive him crazy. Hope it gets better soon!