Gst and non gst firm

Yes you can open another business but if turnover exceeds 40 lakh rupees then GST should be registered in the name of the business otherwise if it is less than 40 lakhs rupees then is not required for GST registration and the business should be in the name of his son there is no problem

There are two options depending upon the scale of business and turnover:
it is normal to provide threshold limit for imposition of any tax so that very small taxable persons are out of tax net.
very small taxable persons whose total turnover is less than 20 lakhs are not required to register it is optional .
Sec 22 (1)
Here the business turnover is 40 lakhs so liable to be registered.
He can have one registration number under one company having two business verticals.
I hope Mr. Jadojia your query has been answered.
Thanks

Agree with sourav