Went through exactly this with DS1 (now 5) around the 14 month stage. Was bloody exhausting, and you have my absolute and every sympathy.
I can vividly remember spending 3 hours doing gradual withdrawal in middle of night. We tried co-sleeping, several times, in desperation, but DS1 had never really coslept so this also totally freaked him out and he just screamed. He wanted to be in his cot, but with me in the room, and preferably standing over the cot not sitting down
But I can promise it doesn't go on forever and it DOES get better. If he was a good sleeper before this kicked in, chances are he will come through it and go back to being a good sleeper. DS1 certainly did.
Carry on with the gradual withdrawal - it works, and it's the best way to get through this without feeling you are being either too tough or too soft. Does he have a comforter of any kind (muslin, soft toy etc)? If not, try introducing one; a cuddly toy really helped DS1. I found it also helped to start using the same phrases on put down (i.e. mummy's here, mummy loves you, mummy see you in the morning etc etc) and slowly, slowly it worked. It did take weeks, rather than days, but it worked.
You will find that as he gets older there will quite likely be other periods of separation anxiety type stuff - DS1 went through the fear of the dark phase just before he was 3, for example, and again was unsettled for a few weeks before he calmed down again. But honestly if you are calm and consistent they do work through these phases (and then move on to the next one )
I'm so looking forward to the separation anxiety phase with DS2 (7mo) - NOT