Tier 2 (general) PBS dependant visa - Surname addition

Dear Sir,

Currently I and my family are living in UK with my Tier2(General) and my family''s PBS (dependant) visas.

But my son (4yrs old) Visa doesn't have surname as we got his Indian passport without surname.

In this scenario, we are thinking to add surname (my name) in his passport and also to update in his visa.

All of our visas are due for renewal by June 2020.

In this condition, if i want to add his surname in his both passport & visa, how I have to proceed?

Kindly advise.

Thanks
Venkat