There is good recent research to suggest that infants actually have a 'fourth trimester' of development. Their brains aren't completely developed by birth and that fourth trimester in the world helps 'finish off' the major development of it - though it continues to develop until their 2 and throughout life.
Their bodies are 'finishing off too'... their digestive system, respiratory system and circulatory system is finalizing growth in that last (4th) trimester.
A baby is unable to 'manipulate' as a toddler would, or even as an older infant (say 9 mo) might. All they are doing is responding to a stimulous. Following a pattern of reaction. IE pavlov's dog. Ring a bell, give dog treat over and over again. Then ring bell, no treat, but dog still salivates.
You and he are establishing a pattern of care and love. His cries communicate a certain feeling to you, and you respond by offering food, diaper change, or just hugs. He needs to feel confident in that pattern. He needs to know when he cries, that will follow a certain response from you. Consistancy is EVERYTHING with babies.
He probably is having seperation anxiety. Just think when he was inside you it was 24/7... If you held him constantly or near constantly for 12 hours a day that's still only HALF of what he was getting inside you. Does that mean you need to be johnny on the spot for every beck and call? No, but make sure you are establishing a good and caring consistant response to his communications with you. Sometimes, older infants cry simply because the pattern has changed and they don't seem to know what to do!
