It can be hard can't it!?!
My suggestion would be to use a gradual process. Work out what works best for you both but maybe you could try something like:
feeding him (or rocking him) until he is drowsy then putting him in the cot and shhh/patting/singing until he goes to sleep. If he cries (as oppsed to grizzles) then pick him up, rock till drowsy then put in the cot. It may take a few goes before it starts to get better.
Try that for a week or so then maybe just feed/rock for a little while then put him in the cot awake and rock/pat to sleep.
Over time gradually withdraw contact.
A couple of things that I have found easier with Jack is that I started with bedtime first and tried the new routine for a week and used the 'old' methods during the day. In the second week I introduced the new routine to daytime naps as well. I found that at night he was tireder, the room was dark so less distractions etc so change was alot easier to instigate. We are in our second week of settling in his cot and so far we have been doing pretty well. Also this time I told him when we started that he needed to go to sleep in his cot and that I would be right there with him. I know he is older than your son and I am not sure whether he understood me but I felt like I made him part of the change rather than pushing the change upon him.
Good luck