I suspect my ds has ADHD - what do I do?

[Carrotsandcelery]

I met with my ds's teacher and headteacher today to discuss his behaviour. During the discussion the headteacher asked if we had seen the doctor with regards to his behaviour. We then discussed his levels of anxiety and methods of controlling it and his impulsive behaviours.
Due to the comment about discussing it with our GP I have been looking at anxiety in children on the internet and ADHD seems to appear again and again.
Looking through the list of symptoms on these sites he "ticks" the vast majority of traits.
What do I do now?
What are other families' experiences of investigating such symptoms?
What does an ADHD diagnosis mean for my ds?
I have so many questions and don't really know where to start.

The additive side of things is very interesting as my dad is very reactive to additives (he will be 70 this year!) and has to be careful.
What additives should I avoid?
Ds has had a number of medical problems up to this point, one being a problem maintaining his weight. Our dd who is older eats a very healthy diet but ds gets more junk as we were told to give it to increase his calorie intake.
I will have to switch to healthier high calorie food I suspect.
He doesn't like fizzy pop so that one is easy but he does get more chocolate than anyone health conscious would ordinarily give a child for the reasons stated above.
I will have to look into the sweetners too - I know they pop up in the most surprising places.
Thank you so much. Please keep info like this coming in. I have been scouring the web but there is a lot of negativity out there so this feels much more positive.

I happened to see the GP today and he told me he had spoken to the school doctor, written the letter and that they were obliged to see my ds within 12 weeks.
He is a lovely man: he was concerned about the impact of the wait on me, dh and dd, as well as obviously ds himself.
I got more grief from the school today and am growing concerned that ds is being labelled the "naughty" boy and that other kids can get away with blaming him for things.
For example he was in trouble today for pushing 2 children - unacceptable behaviour that has to be tackled.
However, the first child he pushed had been laughing at him and the second child had been pushing him first but they have gone unpunished.

I am not sure the girl laughing should be punished but the boy who pushed him first surely should be if ds was?
Does this sort of thing happen a lot once kids realise a child is easy to blame?
Any suggestions how I tackle this with the school? I have explained what the others in the class were doing in his home/school link book. Is that enough?
We are so stressed and so heartbroken that ds is facing this negativity on a daily basis.