I want to travel to Oman with family but my son passport expires on 20/03/2018
travel will be 18-29/10/2017
how Oman immigration is with visa on arrival? I know passport should be valid for 6months.....
Before you meet the immigration officer your son faces the challenge of being allowed on the aircraft.
You say you
know the passport should be valid for 6 months. It won't be. So getting a new passport would probably be a smart move.
You have plenty time, and some of the life of the old passport will be added to that of the new one (I think up to three months).
Any reassurances or promises you receive here matter not one jot.