Hey N.,
I got into a bad habit of rocking/holding my daughter until she was asleep before putting her down when she was little (2-maybe 8 mos).
Later when she wanted to be rocked she would cry for about 20-30 minutes and if you went and checked on her she was ok till you left, then more crying.
I started Telling her what's gonna happen "we are going to drink Milk, go brush our teeth, change diaper, sing our song, pray our bedtime prayers and go to bed" She was really Bad about getting to sleep at first so we started on Friday night (we are off on weekends)-that way if we needed we could catch a few zz's the next day. She had 2 nights to try and work out her own issues. She cried on Sunday and Monday nights but after that she was fine.
You also might wanna tell your son you are going to put the rails Back if he can't sleep in his own bed.
One other thing-we had TOO Many night lights in her room. I thought she might have a tendency to be afraid of the dark, but while we had company my husband put all the lights off and she slept Much better.
You might also wanna ask him WHy he wants to get in bed with you-or Why he can't sleep in his own bed. He might be able to articulate enough to get a message across.
Good Luck!!
C.