@skumar,
Apart from what has already been written in the post by Sri Buddy, you need to perform Pitru dosha nivarana puja at Gokarna in Karnataka, Trambakeshwar at Nasik or Siva temple at Rameshwaram in Tamil Nadu. A shanti for Ketu and Venus is also needed to be done at any place near your home. All Shantis and Pujas are to be done with a sankalpa for the child’s speech restoration.