I feel for you OP, my DD went through a phase (for a long time!) when she would only settle if we sat on her bed. But it was after a night terror (about ET, stupid DH letting her watch it!!)
So a bit different as she was genuinely frightened of being alone, but we did gradual retreat. Still takes her a half hour on a good night but she settles herself.
Our problem is that she comes into our bed in the middle of the night and disturbs my precious sleep (I have a 4 month old too)
We're using bribery to try and keep her in her bed. She stayed in her bed all night last night so we made a big fuss about her and bought her a toy. She's gone to bed clutching the toy and we've reminded her why she got it in the hope it might keep her in her bed!
Sorry for the long waffle, no real advice for your exact situation but good luck!!
He has trained you very well. Now it's time to take control and train HIM. No more laying down with him, no more giving in to his every cry. Put him back into bed and sit outside his door until he is asleep. Tell him you love him but it's time to sleep. Tell him you will sit at the door but you are not coming in and you will put him straight back into bed should he get up. Then you have to follow through. It might take hours and it may take several days, but he will break out of his current cycle.