Divorce application under 13(b) pending can i put 13 (a)

No he has to withdraw the present one and then file a fresh case 

No, your son has to firstly withdraw the mutual consent divorce petition thereafter he  can file new divorce petition .

Peition u/s 13B(1) of Hindu Marriage Act, 1955 (first otion of mutual consent divorce) is stated to have been disposed / allowed.

Consequently the husband (your son) has moved in another petition u/s 13B(2) of the Hindu Marriage Act, 1955 (for second motion of mutual consent divorce) within stipulated period of 6 to 18 months of first motion, wherein  his wife (your daughter in-law) did not appear, when the petition was filed i.e., on or after 26.05.2025 and the court has fixed next date as 20.04.2026, isn't it ? If so, let you and your son wait for the date fixed but no other application shall be entertained in the case.

However, after dismissal of the petition u/s 13B(2) of the HIndu Marriage Act, 1955, your son may withdraw the case and move fresh case u/s 13A(ia)(id) of the Act or any other relevant provision of law for divorce on the ground(s) as applicable in his case, as opned and advised by learned senior experts herein above, which I endorse.

What is the opinion and advise of the lawyer engaged by you / your son ?