lorraine, that is an excellent question, and quite frankly I don't know.
I did find this, however, which is somewhat startling:
A minor cannot consent to treatment for another minor, that is a mother who is under the age of 18 years cannot consent to her child's treatment/surgery. In this event, the person with parental responsibility for the mother would have to consent; this should be documented in the notes. If this was not possible, in an emergency, then in the child's best interests the hospital barrister should be contacted, as to how to proceed.
http://tinyurl.com/3njgc7
This would indicate that a 16 year old mother does not have full parental responsibility, and the same would apply re your son.
The question now arises - in the hospital scenario would you, as having parental responsibility for your son, have the same rights to consent to treatment as the maternal grandmother?
Thank you for the question, and sorry I can't answer.