When my daughter started waking up more frequently at night even when she didn't need to nurse, I knew it was time to put her in her crib and keep her there. In my head, I was doing it as much for her as I was for me. She wasn't getting the quality sleep I know she needs in my bed with me.
BTW - I coslept with my daughter and have had no trouble with teaching her to sleep in her own bed. It's not the cosleeping that creates this problem...
Will his waking actually wake your husband up if they're in different rooms?
As his mom, you can tell the difference between his "I'm scared/something's wrong" cry and his "I'm lonely and you always come in when I cry" cry. Leave him alone when it's a lonely cry. Give your husband earplugs as someone else suggested, if necessary.
I would also recommend starting a bedtime routine that does not involve him falling asleep with your or your husband. You can do it gradually. Start putting him down when he's *almost* asleep, but not quite. Do that for a few days/couple of weeks. Then bump it up to where he's fully relaxed but not dozing off/almost asleep. Eventually, bump up to putting him down when he's acting sleepy, but not fully relaxed. Then, put him down after your routine is finished.
My daughter's routine is bath, brush teeth/hair, get dressed, read book, kiss and hug, then crib.
If he cries, you'll need to let him cry. There will be times when he won't calm down and you'll need to get him and help him calm down, then lay him back down in his crib. If he sounds to be calming himself, even if he's still crying, let him be.
I thought it would be way more difficult than it was for my daughter, but she caught on pretty quick. Most toddlers do. It's us as the parents that tend to fight changes and make them harder than they have to be.
Edit: About the co-sleeping vs. cry it out. At this point, your son knows he can trust you to be there when he needs you. He's taking advantage of that. Just because a child cries and you don't respond immediately doesn't mean the child will start to think you aren't there for him. I don't believe in cry it out when they are very young - they need to learn the trust. But, at some point, children know their parents are there for them or not. Once they trust that their parents will respond when they truly need them they start to try to get them to respond when they only *want* them. And, of course, we need to respond to those wants, too - just not in the middle of the night. At night, we sleep - that is what they need to learn at your son's age, if not before. Letting a child cry because he or she *wants* mommy b/c he or she is bored or lonely or just pulling their "let's see what happens" strings is not cruel or damaging. However, if you have a really needy child and that child still does not trust that he or she is safe if mommy isn't there, then crying it out is probably not the best option. That doesn't sound like what your son is dealing with.