I would definitely ask about the MRI. Does the Dr. know you are a nurse? My kids pediatrician knows that I am an RN and is very open to my input, as a nurse and as a mom. I think most good pediatricians would, and should, be open to all questions and input from parents, you know your child better than anyone.
you did what was right. you can't make the Dr do anything else. the only other thing I would add is to notify the nursing supervisor at your hospital. Had a situation once with a pt. who had a ruptured appy, the Dr wasn't going to take her for emergent surgery, so we called the supervisor. The patient ended up with a new Dr and surgery after all.