This is called separation anxiety. It is not separation anxiety disorder. Separation anxiety is common to many children, especially those who tend to be shy or more introverted. If you google the phrase "separation anxiety and young children" or "separation anxiety" or "shyness in young children" you should be able to find many ways to help your child. But really, the best thing you can do for your child is over-socialize him - take him to McDonald's, the park, the mall, grocery shopping, visiting friends and relatives, have many playdates, church, the beach - whatever. It may take quite a few months before your son interacts with others, but the many instances of being around other people will help lessen his fears and help him realize that other people (besides Mom) are also "safe". All the best ...
Does he attend daycare/creche/kinder?
My stepson was very similar, even though he is a very clever little boy.
We enrolled in him in creche, begining with a half day then gradually incresing it to a full day and he now attends twice a week.
For a long time, he would cry and scream when we left and also cry when we picked him up.
But, we rode it out (as hard as it was) and he is now happy to go, knowing that he will have fun.
Not only has this helped him with his separation issues, but it has also improved all of his skills and manners :)