My ten-week-old son cries whenever I put him in his crib. He will sleep in his bassinet or in the car seat carrier, but starts to fuss immediately in the crib. How can I get him to stay and sleep there? Is he too young to "train" to sleep? When should I start and what steps do you recommend? I am particularly interested in whether or not I should let him "cry it out" for a period of time.
Some young infants seem to need a "snug" environment in order to feel comfortable enough to fall asleep. Because there's not much extra room in a car seat carrier or bassinet, these may be more conducive to sleep. The crib may feel too big and open for some babies. Or it just may be that he's gotten used to the bassinet over the past ten weeks and objects to something new. As babies get bigger, most adjust quite easily to the crib, and I would recommend that you keep trying. You might want to put him near one end of the crib to make it seem more compact. Also, make sure the crib is not cold or drafty; it's okay to have a bumper inside the crib. You can also try patting or rubbing him on the back the first few times, in order to help him settle. Don't get trapped into doing this for too long though, or you'll have another habit to break later.
I would not recommend letting him cry it out at this age. Young infants (up to four to six months of age), usually cry for a good reason, even if we can't figure out what it is. I think infants this age need to be comforted when they cry, and not left alone. You can still try to train him to sleep better by trying to set a regular eating, sleeping, and activity schedule, and by recognizing his cues that tell you when he's starting to feel tired.
When babies are older (five to six months), I would start using methods that allow them to cry on their own for progressively longer intervals. Dr. Richard Ferber's book: How to Solve Your Child's Sleep Problems has excellent chapters that explain how to assess your baby's sleep, and how to implement a training program.