For the purposes of source of payment for the acquisition of property, you will have to justify from whose income the same was paid. Mere payment from someone's bank account does not qualify that person to be the asses able owner under income tax laws. In case the payment was made from the income of your son, TDS and other tax liabilities shall accrue to your son who is an NRI.
However, if you can justify that the payment you are talking about have been made from your own independent source of income, then you will qualify as proportional owner for the purposes of income tax.
In the first case, assuming all the cost has been paid from the income of your son, TDS shall be deducted as per Section 195 (applicable for NRIs @20%). In the second case (means proportional source of income) TDS shall be partly applicable for NRI as well as Resident.
As such, to be able to calculate the exact amount of TDS, Cost of property, source of income utilized shall be required to be known.
In addition to this, if you sell the house (whether at gain or loss), same rules shall apply as to what was the source of money for the payment of cost of the property. However, Stamp Duty and other taxes shall be considered as part of cost of acquisition of the property.
As per the Income Tax Laws, source of income your acquisition of any capital asset is very important. Further, with the amendments in the Benami Property Act, the same has now become much more critical.
I hope the same is clear to you.