It takes time to create another pre-nap routine for him...
he misses you, and so he cries. He's used to co-sleeping...
Do you put him down for naps at the same time each day?
Do you nurse/feed him before hand?
Do you let him wind-down first, before putting him in the crib?
Do you make everything quiet, before nap?
Does he have a 'lovey' to sleep with? or does he take pacifiers?
For me, this is the way I got my kids to nap when they were babies... it takes me about 1/2 hour of winding-down my kids before I actually put them down. We have the SAME routine everyday, before nap. My son knows it like auto-pilot. He at his age will even tell me sometimes when HE wants to nap, when it is time.
But the thing is regularity... and routine. Everyday the same. Then, the child "knows" what to expect.
My daughter was not as easy to put to nap, so I'd lay down with her... she'd nurse, then fall asleep, then I'd get up and go do whatever I had to. Other times, I'd put her in the crib... and I had baby safe toys in there, and she'd play around in the crib first, then slowly she'd fall asleep on her own.
My son, is more flexible and easier to put to nap... but mostly because I did the SAME routine with him since he was 6 months old. So now at 2.5 years old, he STILL naps and we have the same pre-nap routine. But at night only, we co-sleep. So before nap... I tell him verbally "Nap time soon...." then I get him ready, we watch a Little Bear video or I read to him, I change his diaper, give him some milk, I let him just wind-down, I tell him "quiet time..." and then I tell him "okay let's go..." then I put him in the crib. He plays around with his toys first, talks to himself, then falls asleep. He also has a 'lovey' that he sleeps with. It's his pal.
But each day, I do the SAME routine with him. And when my daughter gets home from school, the afternoon is ALWAYS "quiet time" and they know it... then my son goes to nap, and sometimes even my daughter will fall asleep.
But the MAIN thing being, no matter what your "routine"... do it the same everyday, the same time, the same sequence... and then the child gets used to it.
I never let my kids cry it out... but just got them into a routine.
At the age your son is... what I also used to do with my son, is I'd sing him the SAME song as I carried him/rocked him before putting him in the crib. So he got used to that... the same song, then he goes in the crib. And then I'd turn on a fan on low... for white noise. So now, if I even forget to turn on the fan, my son will tell me.
So it's a pattern and a routine... that helps the child get into a nap "habit" and then less protesting.
Anyway, just some ideas on what I do.
I know it's hard... getting a child to nap... but keep trying... a baby needs to nap, it's healthy for them. See what routine will work for your baby... then keep it the same everyday.
Al the best,
Susan