Hi,
Welcome to ehealthforum,
It is difficult to comment on the cause for the bruise on scrotum that you describe without examining it. If it is getting painful, or if he is developing any other new symptoms, he should not delay in visiting a doctor/healthcare provider for further examination and evaluation. Avoid activity that aggravates symptoms. Treatment as indicated (based on examination findings and investigations results) would be started to help him.
Take care.