I'm quite new to this too, but my LO did the same thing. Do you make eye contact with him when you're trying to wind him down? I know with my DS if I avoid eye contact (as hard as it is) that he settles down a bit. Also, perhaps he's not tired? How long is he awake before you're trying to put him down? I know the activity time is supposed to increase around 4 months, so maybe that's the problem? But then, one would think you'd be having problems with naps too.
Just a few thoughts...I hope they help!