i dont think you should just leave him crying until he falls asleep. the easiest thing to do is to swaddle him (wrap him in a blanket...but not toooo tight), give him a pacifyer (only when he sleeps at night and naps...not during the day), and put the radio on very very quietly (talking or music). When he is wrapped in the blanket (should be thin cotton and very light so he doesnt get too hot), has the pacifyer in his mouth, and the radio is on, lay down next to him on the bed as he falls asleep. Close your eyes and let him see your face so that he is tempted to imitate you and sleep as well. If he doesnt want to try and sleep at first you can calm him by reading a book while hes bundled in his blanket with his pacifyer. If you do this as a routine, eventually he will get used to using these signs to fall asleep. The soft radio (especially ppl talking) is good because it mimics sounds he heard while in the womb.
Some people say swaddling should stop when the baby is very young but others say just continue if it comforts the baby. Only swaddle when hes ready to sleep or nap..not during the day when he should be running around, of course. Also, make the blanket tight enough so he feels secure and is bundled, but dont make it too tight. make sure that as he is sleeping, he can loosen it and even remove it as he moves in his sleep. Its ok if he removes if completely as hes sleeping.
Before you swaddle him you can also do something soothing like read to him or give him a bath. Basically he needs a routine that gives him cues that he should be getting sleepy.
After he falls asleep in bed with you, just gently put him in his crib and you can leave the room.
Another key is that whoever puts him to sleep should be there when he wakes up. That way he wont feel like he will be abandoned while sleeping. So for example, if you have a babysitter, this person should put him down for his nap and also be there when hes awake. You shouldnt put him down to sleep and then have a babysitter arrive as hes still sleeping so youre not there when he wakes up.
I guess everyone has their own method, but this really works! Since you may not have been doing this since he was little, you might have to get him used to it but after awhile he will sleep so easily. Maybe eventually you can stop swaddling him but do the other things so that he can get used to falling asleep without being bundled.