If you (and your SO) are enjoying the co-sleeping, you don't NEED to transition him to his crib. He's still very young, and it's normal and completely natural for him to want to be with mommy. I know plenty of happy mama's and babies who are still co-sleeping well into toddler-hood. There's nothing wrong with it. If however, you're wanting to move him out of the bed for other reasons (which is also totally normal and natural--mama's gotta have some space of her own
), then I also suggest doing it very little, by very little. *I* wouldn't let him CIO, if it was my baby. With my first ds I did use this method because I thought it was the "right" way, and I figured that he had to eventually learn to go to sleep on his own, nap when I wanted him to nap, etc.. But I looking back I feel absolutely terrible about letting him cry. And I won't let my ds2 (who's now 9mo) CIO. It's pretty harsh when you think about it, and there's a reason that you feel terrible listening to him crying all alone--it's just not the natural way of things. Not to mention, if you answer his cries ASAP, he'll only feel more secure in knowing that you're not far away. But anyway, not trying to get preachy--just my 2 cents.
With that being said, if you want him out of the bed sooner than later, I would perhaps try putting him down in his crib for naps when he's VERRRRY tired, almost asleep. If he wakes right back up, go through the routine again (do whatever you normally do to put him to sleep, then transition him to the crib). Eventually he'll realize when he's waking up there, that's where he's supposed to sleep, and as he gets older, he'll get used to it. But I think that very slowly, and not just leaving him in there all by himself, is going to be the most gentle way. If he's not used to it, he's not going to want to be there, particularily if he's "forced" to be, KWIM?