As you have said that he is residing in new house with a lady and children. You can have ample materials as an evidence, such as affidavit of neighbours, the agreement of the flat, school documents of the child, if the child is in school or birth certificate of the child.
Furthermore, if the Spouse is alive then it is not permissible to do second marriage without obtaining divorce, hence it is a punishable offence. Thus, you can file FIR against him for his wrongful action.
Hope you have got solutions to your doubt. For any furthe clarrification, you may call on below given number.
Adv. Rohit Dalmia