funnyonion - may I refer you to an excellent BF website which is based on scientific research ? www.kellymom.com (sorry, if you know it already).
Does your (moron) GP realise that breastfed babies do not all follow the NHS charts given to mums in the "red book" ? That is because the NHS charts are not based on exclusively breastfed babies who tend to weigh less than formula-fed ones. I think it's outrageous to tell you to give him formula at 11 months when he has done fantastically well for such a long time on BF.
May I ask what he weighed at birth ? BF babies should double their birth weight be 5 - 6 months old. By 1 year old they should be 2 + 1/2 times birth weight. If he is within that range, I would NOT give him formula.
My DS is now nearly 9 months. At around 5.5 months he dropped down from the 25th centile on the NHS chart. If I look at his weight now (7.8 kilos) he has fallen way down the NHS chart. So I looked at the World Health Organisation (WHO) chart on the website mentioned above which is a chart for breastfed babies. Guess what, my DS is and has been on or around the 25th centile on the WHO chart since birth, i.e he is spot on as far as I am concerned.
So print off the WHO chart, give it to your GP and tell him to put that in his pipe and smoke it !
I told my HV that I refused to go by the NHS chart as it's based on a mixture of formula-fed and BF babies, i.e it's not suitable for babies who are exclusively BF for the first 6 months like my DS. She said, "no way", despite the fact that I showed her the WHO chart which looks completely different to the one in the red book I could have punched the woman in the face ! These people are just in total and utter BF facts denial !!