Question on serving summons -

Find out the latest address otherwise even if you get succeeded there will always be a chance to get the decree obtained by you set aside. Service on last known address is also valid and service of notice through publication is also valid but since he is out of India so there is every chance that he may get decree set aside by just producing his passport entry to show that no notice was sent or served upon him. Even on visit the process server and post office in their report may give report that he is not residing at given address and also that he has left for outside India. So act prudently.

I agree with Adv. Siddharth Srivastava. You can serve the summons on last known address. In addition to sending the summons to the defendant via process server, you can apply to the court for copy of summons under Order V Rule 9A of the Code of Civil Procedure Code for service of summons shall be effected to or on behalf of such plaintiff by delivering or tendering to the defendant personally a copy thereof signed by the Judge or such officer of the Court as he may appoint in this behalf and sealed with the seal of the Court.

If the rules permit within your state, then summons can be served via email also.
Refer to the provision for service of summons in case the defendant resides outside India as per Order V Rule 25 of the Code of Civil Procedure.

Regards,
Tushar Jha. (Non-advocate)