The legal status of the person belonging to the STs doesn’t change becasue of conversion but the same is not true for the SCs.There are instance when SCs register themselves as Hindus to avail the benefist even though they have converted to Christians. He may come under such category those of whom are called crypto- christains. A case of criminal misrepresentation will only taken if due to act of the person you have suffered wrongfully due to such concealment. It is covered under section 107 of the Indian Penal Code. Other than misrepresentation, you can also file a case of cheating under section 415 of the Indian Penal Code.
Cheating comes under property offences, as he had demanded dowry which itself is a offence you can add go under specific section of section 419 that is cheating by impersonification or 420 which is a aggravated form of cheating. More than misreprensation cheating would covers wider wrongs such as harm to body, mind or reputation. It would be advisible to go under cheating. From the given facts only this much can be said with certainity. As there is already an offence of dowry, it is better to add these offences or offence to the same First Information Report.
Reference: Indian Penal code 1890