It sounds like, and is the right age for, separation anxiety.
The most effective way I have found to deal with this is to give lots of reassurance, then gradually reduce the amount of reassurance needed. Leaving baby to cry is only going to make him insecure.
I would set up a mattress on the floor. Or if you are comfy, just a chair by the cot. Keep establishing that sleep time is for lying down, still and quiet. So every time he gets up, lie him back down. Every time he shuffles, place a firm hand on his chest/back to still him. Every noise gets a shushhhhhh in reply. Also I think comfort needs compassion too. I don't buy into this idea of ignoring the child (well, not until much older anyway). I would give lots of eye contact, lean into the cot to give plenty of physical contact.
Constant reiteration that you will be there to give as much comfort he needs, but there are rules. He must lie still and quietly and go to sleep. A dummy or comforter toy may help as sleep triggers.
Once he accepts the rules and trusts that you are always going to stay to give him comfort, it stops being a battle. There will be no fighting to stay awake. He'll start to learn 'the drill'. So you should need to do less actively reassuring him, maybe just lying/sitting/standing nearby to feel secure enough to sleep.
That's when gradual withdrawal starts. You could start gradually moving further away. A little more each night/week. Until he just needs a door open and to know you're just outside the door, but will come if he needs you (you don't need to be waiting outside his door really, he just thinks you are. You could be in bed. You just need to respond quickly if he calls.).
Then the final step is closing the door - but with him trusting that he's not separated from you, you are still there, he doesn't need to feel anxious because you'll come straight away if needed. But once he's settled, he should feel secure enough to then let you leave.