ANSWER:

You father can make dua for a few seconds after the imam has given tasleem but he shouldn’t take more than this much time.
He can make dua in his sujood and before tasleem in his voluntary prayers. 

https://youtu.be/hYToZVeAl20

my father said that he can’t able to make dua behind the imam since the time is very less.usually my father use to make dua ayleast 10 to 15 minites.the imam gives tasleem before he complete his full dua.what is the ruling for making dua in thashahud behind the imam.i hope u understand my question.