Thanks for writing your query.
Normally blood should be absent in urine.
Even microscopic hematuria
i.e. blood seen during microscopic examination of the urine should be thorougly investigated.
You can repeat his urine examination and if pus cell are also present, get a culture/senstivity done of his urine.
Blood in urine can also be due to stone in the urinary tract
and should be investigated by X-Ray KUB region or Ultrasound
KUB region. Pain in abdomen can be caused by a stone in the urinary tract
Watermelon if not properly digested, can present as red particles in stool.
You can look again after a day or two and if you are not sure get his stool examined for Routine/Microscopic and for Occult blood.
Treatment can be planned according to the reports of the investigations.
If the problem persists, get him examined by his Paeditrician.
I hope this is helpful to you.