DD went through a stage like that, she would just keep standing up and then eventually start crying because she couldn't work out how to get back down again. I started by going in the first couple of times and lying her back down while saying to her it was sleep time and she needs to lie down. Then if she kept doing it, just walked in and without making eye contact or talking to her would just lay her back down again.
Now if she stands up (which rarely happens) I just walk in and ask her to lie down and she usually does. It will get better, so I would keep persisting with what you are doing and he will eventually get the message!