My son of 8 months is not having one ball in the scrotum. . Advise up to what time I should wait that it will descend automatically to the scrotum. Alternatively, up to which age it is safe to have surgery and get the testis descend to the scrotum.

That is something that you need to discuss with your pedetrition or family doctor.

Testes usually descend before birth, however, in cases where the testes have yet to descend, they can take up to about a year to descend... so at 8 months, you still have some time.

As far as surgery goes, as Jesushelper says, it really depends on what your pediatrician would recommend.

If it does not appear even at 3 months after birth it will never. It needs to be located and if found brought down before 18 months. Sometimes it points to the presence of other abnormalities.
So see a Paediatric Urologist or a Paediatric Surgeon.

My brother age 18 is not having one ball in the scrotum. Please guide me what action would do for his future.