Put the child in the playpen at a time you don't need him to get up. Tell him to get up. If he does not get up then come back in 5 minutes and tell him to do it again. Keep trying and don't let him out until he gets up.

Start in the morning after breakfast and dont give up. He will stand up by lunch. If you have a camera put it on him so you can see right when he gets up and get him right away. He may drop as soon as you open the door. If he does then go to him and tell him to stand up. If he refuses then start over.