Thanks for your query,
Acute retention of urine can be caused by
infection,calculus,meatal stenosis,anterior and posterior uretheral valves,blood clot,
he needs to be evaluated,urine routine examination,urine culture,CBC,ultrasound
abdomen needs to be done.
I will advise you to get him evaluated by pediatric urologist
,he needs to be investigated and managed accordingly.
Wishing your son good health