Court has power to allow the application if it find that calling witness is necessary for the case

 You can object on ground of dealy and that witness was not in list.

You should object to examination of son as witness when case is kept for orders after evidence has been recorded of parties 

It depends on the role of the witness in this case. If he was important why he was not examined earlier. You can object to the same on that ground

Application under section 311 can be filed at any stage. 

But you have to oppose that application on the ground that they want to examine at this to fill up lacuna of case and purpose of 311 is just for decision of case not to fill up lacuna. 

If you want that court will not allow the application under section 311 CrPC then you have to file reply to the application with objections that this application is just made for delaying the proceedings so that complainant can harass the accused.

1.  A party to a litigation can furnish as many witness as he wants and there is no legal bar that the part can not bring more than one or two witnesses to corroborate his evidence.

2. A party can lead evidence through his family member as well and law does not attach any defect or impurity in such testimony.

3. So in your case the court has committed no error in law or in fact in allowing the son of the complainant to adduce evidence in court.

4. The defence of the accused person would be fixed after getting the evidence what the witness has adduced in his examination in chief.

311 Crpc Power to summon material witness, or examine person present. Any Court may, at any stage of any inquiry, trial or other proceeding under this Code, summon any person as a witness, or examine any person in attendance, though not summoned as a witness, or. recall and re- examine any person already examined; and the Court shall summon and examine or recall and re- examine any such person if his evidence appears to it to be essential to the just decision of the case.

Hi

It is court's discretion to allow / reject the application. Other way round, going by the situation, if his presence is going to help you, you request the court to allow the application.  It will be done without any delay.

The probable reason for calling the son of complainant is to fill any scope / gaps which the first witness has give you.

Take advantage of the same and take maximum leverage of the situation.  

Examine him thoroughly.

Good Luck 

1. an application U/S 311 CrPC can be produced at any stage of case,

2. there might be some lacuna in the case, which they are trying to fill through the witness of son, 

3. you are advised to oppose the application by filing a reply based on facts of the case

Court at any stage of any trial summon any person as a witness if his evidence appears to it to be essential to the just decision of the case. And when you have not objected, court may permit the applciaiton.

IF you have objected it than court could have decided on merit whether his presence is necessary or not.

There is no legal infirmity in roping in additional witness by the complainant at any stage except if it is done to fill up lacuna.

As a defence you may record your objection and file a counter to the petition filed by them whereas you preferred not to do it, hence you cannot object to it now.