Thanks for your query,based on the facts that you have posted it appears that your son sustained a trivial injury
over groin and private parts and now has painful urination .
Most likely this is due to trivial injury to penis.
Since he does not have any bleeding from urethra
There is nothing to be worried about
Please give him broad spectrum antibiotic like Cefexine along with anti inflammatory drug like Diclofenac
twice daily for couple of days.