You can apply for British citizenship after 5 years of exercising EU Treaty Rights + 1 year so 6 years in total. So assuming you have been exercising treaty rights from May 2005 to May 2010 (for example as a worker), you could apply now for British citizenship if you meet the
Standard requirements for naturalisation and passed the Life in the UK test.
As for your son, he will be automatic British citizen, if you have gained Permanent Residence in the UK (automatically after 5 years) before he was born. So it depends when exactly in May your son was born - whether before or after your 5 years. If After the 5 years, then he is already British and could keep his Dutch nationality. If before the 5 years, you can now apply to register him as British. See
registration. If you do this after your son has been living in the UK for 5 years, he will not lose his Dutch nationality.