Hi Katie
First thing to check is that he can get himself down. Spend a bit of time with him in his cot, moving his hands down the bars so that he learns how to sit himself down.
Unfortunately then on its just being consistent with the re-settling. For us, and for most I think, its a developmental stage. We had night wakings at learning to turn, learning to crawl, learning to stand (I'm sure our next one is when he learns to climb out of his cot :roll: )
So be consistent so as not to start any bad habits and it will gradually stop. A lot of the time, they stand up in their sleep. I think because they learn by instinct and not by being taught, because they can do it, they do. KWIM.
I'm sorry I've no magical solution but it does go away after a couple of weeks honest.