Can you legally be held responsible for a medical debt for your child?

Here's the backstory..I was gone on deployment when this whole thing went down but my son was sick I guess and my EX (his father) took him to the doctor's and they billed me for it and since I was gone I had no knowledge of it and it's not on my credit report affecting my score badly. We have tricare health insurance but I guess it wasn't used or it didn't cover whatever was done. My question is, how am I legally responsible for the bill if I wasn't the accompanying caregiver at the time? My son's father was and they share the same last name (I do not). Is this legal? I asked his father to pay it but he won't and feels it's my responsibility. I will pay it off but first want to know if it's legal what the collection agency is doing. I've called them twice today but keep getting hung up on after b being transferred.