You would think they would treat it aggressively in a 7 month old baby?? I really don't have any knowledge of this, but that is what they would do for Psuedos...so seems like a no brainer for MRSA. Seems like you would want to try to get rid of it right away if possible. I would push to have it treated...
Also, don't blame yourself. Sounds like you are taking all precautions possible to keep him safe. He could have gotten it from clinic, who knows. Be very careful when you go to clinic. There could be germs ANYWHERE there! You are doing a good job, keep it up!
Also as suggested, listen to the docs point of view of why he does not want to treat it. If you don't agree with him, I would def get another opinion. This is YOUR child, not his!
Good luck!