Nightime... not so good

Well our good luck with Zack's bedtime has finally worn off. He has done so good with naps in his converted bed. He doesn't even get out, just lays down and falls right asleep.

But lately when we put him down for bed he's been coming right out of his room. I thought I had it nailed today. After 15 minutes he still hadn't came out of his room. But then right when I thought he may be asleep I heard his door open and shortly afterwards he was peeking around the corner. We put him to bed over and over for about an hour; when we finally give up and he lays down with Bob and they both fall asleep. This.is.not.okay.with.me! I don't want him to get into the habit of sleeping in our bed! I even tried to put one of those balls on the door knob to keep him in his room, but he cries uncontrollably! I know we are soft, I should just let him cry but it's so SAD! Once he's fallen asleep I put him back in his room, because that kid moves way too much, and a King bed is not big enough for the four of us (Abbie sleeps at the foot of our bed, Gavin is in a cradle).

Any suggestions? I just worry that this may be him acting out about Gavin. He just wants to be with us at night and when he's locked in his room he just crys while saying Daddy. It's heartbreaking! Which is why I usually cave.