Cass, maybe Oskar had wind? I know the pain comes and goes when I have it.
On the other hand, he may be faking. Maybe he has associated tummy pain with lots of mummy time and sympathy? Byron has definitely been faking and over exaggerating thing lately. He yells out that he needs to do a poo/ wee to get out of time outs or to come out of his room after bedtime. He yells blue murder when he has just bumped his knee so he can have cuddles and kisses. (He could have them anyway if he just asked )
I hope he is ok.