My son stays in USA. He owns a house in India and has let it out. He has asked tenants to pay rent to me so that I can lead a decent life. He has not received any rent. Is he still liable to tax? What if he transfers the house to me?

Your son is the owner of the house and therefore he is liable to pay tax even though you receive the rent. If the house is transferred, then you would become the owner and you will have to pay tax on the rental income.