Son has very bad stomach cramps and is constantly tired?

hi there my 16 year old son has been complaining of stomach cramps since this morning and all weekend he has been so sleepy and tired even though he was in bed most of the time i rang his doctor and she said she wont be able to come until tommorow morning .i thought about bringing him into hospital but he will not go he is petrified of having to get an injection .what do you think it may be