co-sign for cellular contract

What is the name of your state? California
I was being too kind to my ex-friend by helping him out by signing a cellphone service under my name since he had no credit. He has been using the cellular service under my name over 4 yrs and even uses the number for his business with his business cards. The billing address is also sent to his home address. Since business have not been good for him, he has stopped paying for it and the bill of 580.00 went to collection of which I had to pay. If I have proof that this line was being used by him even though it was under my name, do I have a case to sue him in small claims court? Please Help. I've learned my lesson for using my credit.

I hope you've had the cell phone shut off.

Take him to small claims court. And never be foolish enough to do a "friend" a "favor" again. The fact that he had no credit should have put up a HUGE red flag.