Your son being a biological son of your ex-husband, he has right in the intestate property left behind by his father. Since the house property is jointly owned by you and your ex-husband, you already have 50% share in the said property. Besides your son, your ex-husband's mother (if alive) has an equal share in her deceased son's intestate property. Since you are the mother of your minor son, you have all the rights as his guardian to protect his interests in the share out of the intestate property left behind by his father.
Therefore you should file a suit for permanent injunction against the defendants from interfering in your possession and enjoyment of your share in the property as well as your minor's son's hare in the property, this suit can be filed on your behalf a well as your son's behalf by you.
You should issue a legal notice to the secretary of the society asking him to restrain from transferring the shares of the property to a third person and also stopping him from giving NOC for a sale by any third person. The secretary of the society may be impleaded as a defendant in the proposed permanent injunction suit.