Marie, the biggest bit of advice i can give you as a mum is obey your instincts. You and only you know your boy best and if you are concerned keep going back to you doc and if neccesary ask for a second opinion. Dont be fobbed off if you know there is someting wrong with your son.
Not wanting to worry you, but as Jimbob said our son has problems and i have had to fight all the way to get my concerns taken seriously.
Good luck and keep your chin up!
I'm sorry to disagree with you .. but "mother knows best" is NOT always correct, and can, sometimes be totally wrong

I know of a local case where the mother was "convinced" her son had an eye problem, despite numerous doctors/opticians telling her he was fine, so for a considerable time - 2 years I believe - she dosed his eyes 3 times a day with Optrex - this contains an antibiotic.
The child has recently been diagnosed with aplastic anaemia, caused it is believed, by long term exposure to the antibiotic.
I know this is an extreme case, and there are many mothers who have been proved right when they voice concern ... but not all doctors are wrong all the time ... in fact .. the majority are probably right most of the time.
Just my opinion BTW