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.

I mentioned the student loan because that is technically what is providing his support while he is at college.  So then who is providing support?  Is it him or is it us (we are both on the loan).