Tax payment on behalf of other persons

Dear All,

My brother is paying the vehile/ Income tax by his own bank account online on behalf of other persons and earns a commission  from each indiviual. He receives all the money by way of cash. He lives in Kolkata. I just like to know (1) does he needs to register for providing this kind of business/ service (2) Does he needs to resister under GST (3) what documents does he needs to keep from clients. 

Thaks in Advance    

Bhagat