Your co-signing the loans has nothing to do with whether you can claim him. If he is away at college, that's considered as a temporary absence from home and, if you are the parent with whom he would normally be living, you can include those months as time lived with you.
You are still subject to all the other dependent requirements, none of which have anything to do with his student loans. Go through the dependent questions under My Information to see if you meet all the requirements.
