There is an organisation in the UK called the National Association for Gifted Children. I don't know whether there is an equivalent in the Netherlands?

I don't know what advice or support they may be able to offer to people based outside the UK. I've just looked at their website and a lot of it is restricted access to members only, and you would need to enquire further as to how relevant it is to someone in your situation and whether it's worthwhile joining or not.

It may be worth joining, especially if the school your son attends follows a British curriculum, they may be able to make contact and get some useful information or advice about teaching methods and ideas to integrate into your son's education.

http://www.nagcbritain.org.uk/