Your son had acute onset blood in urine.there are so many causes of it but the most common are-Renal stone,ureteric stone,bladder stone
,may be stone in the urethra,acute post streptococcal glomerulonephritis
and other type of kidney infection
,UTI,ingestion of any drug like aspirin,tumor in the kidney,rarely bleeding disorders.
But it seems to be from renal stone.
Do some investigation like-USG-KUB,Urine for routine,blood for count,urea,creatinine,bleeding and clotting time as early as possible and consult a urosurgeon or a nephrologist
Mild pelvis fullness is a vague term it can not make any diagnosis but in renal stone mild pelvic fullness is seen.And hydronephrosis
means if a stone obstruct renal pelvis outlet then urine can not drain properly and it become swollen that is called hydronephrosis.It is a bad sign as due to back pressure of stored urine kidney function gradually deteriorate.
Dr Sourav Ganguly