Swaddling worked for us, and just gently trying at every single sleep and nap time. If he didn't do it, I held him (and screeched silently to myself with frustration). Eventually he went down in the cot for about one sleep in 3.
I also tried to always send him to sleep by shhhhing and patting, even if I was holding him, and avoid rocking him while I was holding him if possible, so that he associated the shhh/pat with sleep and eventually dozed off in the cot while I was doing it. And I moved from putting him in the cot totally asleep to only just asleep, then just awake, and so on.
Also, something that I only tried a bit later but improved things loads was putting him in the cot while he was awake, with a mobile or just playing with him. Don't know if it made the cot seem more familiar or what?
I absolutely refused to co-sleep (even now I have a baby I'm a v heavy sleeper, and was too terrified to go to sleep if he was in the bed, which wasn't exactly making it a worthwhile exercise), so it was worth persevering for me. However, if you're only putting him in his cot because you feel you 'have' to, and you're comfortable co sleep, try not to worry about creating a rod for your own back and all those other cliches - plenty of people manage to gently move their DCs into their cots at 2, 3, 5, 6 months and so on, without needing to use CC.