There may be two conditions about the purchase of this property. First, that it is your self acquired property and second that your son has contributed in purchase of property out of his own income. In the latter condition your son is a co-owner of this property.
When it is your self acquired property
If you have purchased this property out of your income then you are the absolute owner of this property. When your son had no source of income at the time of purchasing this property then he cannot claim his ownership. A person can purchase property in the name of his family members and it is a legal act.
The nature of self acquired property does not change merely on the joint name of the purchasers in a sale deed. If you have purchased this property however, it is registered in the name of your son, your son cannot claim it as his self acquired property. If your son had no separate funds or that the property was not purchased with money belonging to him, in both conditions the presumption is clear and decisive that it is your self acquired property.
In the present case both the Courts below have found that there was no evidence that Rup Charan had any separate funds or that the properties in dispute were purchased with money belonging to him. In the absence of such evidence the presumption is clear and decisive that they were acquired by the father in the name of the son, and that they were not the self-acquired properties of Rup Charan.
Parbati Dasi vs Raja Baikuntha Nath De
In this scenario you should file a declaratory suit under section 34 of the Specific Relief Act and get a declaration decree from the court that it is your self acquired property. After getting the decree in your favour you can sell to any person without permission of your son.
When your son has contributed in purchase of property
In This situation you can either file a partition suit or get a power of attorney from your son. If there are disputes between you and your son then the chances are very bleak that he will execute a power of attorney. So you should file a partition suit and get division of this property.