I've talked to your doctor about this already and he thinks it is just our child's imagination and not to worry, and in fact I'm not really worried for my boy's health, I'm more curious about how to react, to communicate that I'm taking him seriously, and also because his sister will then often also suddenly 'get' tummy-ache in sympathy.
He will sit down to breakfast and eat a bowl of porridge, or a boiled egg, or toast, or cereal (doesn't make any difference what) and then after 20 minutes or so I guess he will say he has tummy-ache. 15 mins later and it's gone.
I can rub his tummy for him, or give him a warm hot-water bottle and it seems to help. Sometimes I just say he should finish off what he's eating or have a drink of water or something and it will make it go away.
He doesn't cry or really look in pain, except a couple of times when he has held his stomach and leant forwards. This is why I don't think he's faking it, and anyway when he tries to trick us, he is much more obvious than that!
He doesn't get tummy-ache at lunch or at dinner though. Just breakfast. Every time!
And it's not just with me either - he'll tell his mum when she makes him breakfast.
I ask him where it hurts and it's always his tummy, high up. I know when I eat spaghetti (bizarrely not any other pasta) it sometimes give me quite painful stomach ache, which goes quickly.
I'd like to do something rather than ignore it and hope it goes away, but it's quite puzzling.