cpc ex parte

The query is short of material facts. Please post the complete facts.

a person case file aginst 'b' but b not appear in and Court passed aginst ex parte decree afterthat 'b' person die thereafter his son appear in court as a legal representatives .
question arise there whether court allow or not

The LRs of the deceased defendant can file a petition to implead them as necessary parties to continue the case on behalf of the deceased defendant and can file a petition to set aside the exparte order once they are impleaded as legal representatives of the deceased defendant . 

Yes, LR of deceased defendant may substitute him.

LRs of deceased defendant can file petition for impleadment to them in suit in place of his deceased father and also pray  to set aside ex--partr order.

any judgment or authority have available upon it ?

This is position of law, you don't require any judgment to enforce legal provisions applicable to this situation.

The court cannot deny to entertain your petition seeking the relief of getting impleaded as legal representative of the deceased and to continue the case in the place of the deceased party to the suit

I agree with learned senior expert Mr. T Kalasielvan, it is law, which do not require any citation.

If defendant is dead and exparte it is plaintiffs duty to inform the court and to bring LRS on record if he don't inform and obtains decree on deceased person he cannot excute it, if he has got decree before defendant  death, he has to file FDP or execution on dead person then challenge it.