My Son is in the habit of using his mobile phone when there is severe lightning and thundering .Is it risky.?

No.

Think about it. The phone is not connected in any way to the electrical wiring system, where lightening might enter the house. In the olden days of wired telephones, there was actual reason to be concerned. Now, if your son stands outside using his phone, under a big tree or holding a metal umbrella, then you have reason to worry.

Not as much risk as there was when phones were connected by wiring.

Not really. This is the way Darwin intended things to be.