Long term capital gain if one of the party is nri

Dear Experts,

I am purchasing a property where the son is a NRI and father is a indian resident. The property is registered as co-owned by the father and son. While I am pushing for a 20% + taxes deduction in TDS based on the laibility computed by the income tax officer, the bank and the owners are saying that since the father is resident and their bank details + address will be local only 1% TDS should be computed. 

Seeking your expert guidance.

Rahul