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Severe abdominal pain
with blood in the urine point's towards urinary tract
problem. Perhaps this may be a case of renal stones
. Passage of stones through the ureter can cause severe abdominal pain and the trauma results in bleeding.
This condition usually settles with good hydration.
I would be concerned if your son is persisting to have the pain and is still bleeding; test like ultrasound
abdomen and urogram are entailed then.
Continue to hydrate him well with plenty of fluids. He should be alright in a few days.
Hope this helps.
All the best!!