The rental agreement

I make a rental agreement with my tenant with estamp paper of Rs. 50/. In the rental agreement the paper was signed by my tenant son instead of my actual tenant, his son is a student and above 18 years. Is the rental agreement is valid now ?

Please advise now I have to remake a new rental agreement or the same above agreement was again sign by my Tenant which was already signed by his son. What will be the best for legally valid –  ---------- waiting for your valuable advise - Thanks