Flat on rent

Seek the opinion of Chartered Accountant or through any on line forum on Tax Advise or file RTI Application to Central Public Information officer, Income Tax office of your ward seeking copy of all the stipulations for eligibility and for claiming exemptions of house rent paid. 

To the best of my knowledge you can stay in your Son's house and pay rent against your Son's receipt for the same for the purpose of exemption of HRA.  However, youe Son has to add the monthly rent to his income and accordingly pay the income tax.  It is better you may approach a Tax Consultant.