M.M.
I hate to say it but I think you need to start ignoring him! Desperate times call for desperate measures! You may have a few (more!) sleepless nights and your daughter will probably be awoken, but if you keep getting up to get him, he will only continue and this will get worse!!
Mom son has had a few phases of doing this kind of thing as well and we take the "Super Nanny" approach and give him one extra tucking-in, good night kiss, etc... the second time he is put back with a curt, "it's bed-time". Third time is silent... no eye contact, no acknowledging the behavior, etc.
Are you talking with him, pleading with him to go to bed, reading him another story or singing another lullaby? If you don't think you can just ignore the calls for help, think about what you might be doing that he is liking... and cut those things out. No staying with him and cuddling some more, no extra bedtime story, no talking...
Then, when it's NOT bedtime, ask him about it. See if something is bothering him. Maybe a new lovie or teddy bear will help him stay in his bed? A nite-light? Perhaps you can find something to keep him in the bed. Good luck! I know it's trite but "it's just a phase" and he will probably grow out of it if you can be diligent!