You can claim your son if he lived with your for more than 6 months (being away for college counts as living with you) if he is under the age of 24 and full time student;
If his 24 or older, he must have lived with you all year.
Below link provides more details on dependency rules:
http://blog.turbotax.intuit.com/tax-deductions-and-credits-2/family/who-can-i-claim-as-a-dependent-7...
