Just reading your post,
i dont blame you for your concerns about your son, what mother wouldn't.
I think you should try and talk to his school teacher, have a chat, and find out what they have to say. Take note of the day and times when he is complaining about a sore stomach or other ailment. What foods does he eat, does he react after eating something that may upset his stomach?.
And most of all, dont give up, keep investigating what the problem is, by getting a second, third, opinion....and so on, until you are satisfied with a doctor who can give you a better picture of whats going on.
Just some ideas i thought i would give you.
Best of luck
Mel