There are many possible explanations for his behavior. I've seen it many times, and here are some:
1- He believes the company is so messed up that this is his best chance to get ahead (true, sometimes).
2- He thinks you're a dummy and wants to get noticed by the big cheeses, possibly to replace you.
3- Someone told him this is what you're supposed to do to get ahead, and he's just following directions.
4- He has a psychological problem (attention seeking).
Probably, there are more possibilities. I only listed ones I've seen. It's VERY important to figure out WHY he's acting this way. Once you find the reason, you can (re)act accordingly. If you don't know the reason, it's hard to recommend any course of action that's not totally inappropriate sometimes. Any of the aforementioned suggested actions can agravate the problem, if the real reason for this behavior isn't the one the writer had in mind.
If you can't figure out the reason for this behavior (and maybe if you can), you should ask YOUR manager for help. If (s)he won't help you, then #1 may be correct, and you should probably just look for a new job, and let him take yours.
Good luck!