Myself and my wife are in H1B. My son is a dependent of my wife and he is in H4. How to change his H4 dependency on me?

[Raanchi]

Hello, 

Myself and my wife are working in the US in H1B. My son came to US as a dependent of my wife in H4. She needs to go back to India by end of this month since her assignment is getting over. she is planning to come back to US after 2 months as my dependent in H4. 

I want to keep my son with me in US since he is going to school here. Do I need to apply for any conversion for my son to make him as a dependent on me?