Hiii

For SSN Engineering college your son will have to appear in JEE mains exam or TANCET exam to get admission into any course. The admission is not based on 12th class merit but on the entrance exams mentioned above. For the citoff you can check previous year trends but the cutoff can not be predicted as such because it keeps on varying every year based on many factors like difficulty level of paper etc.
To know more refer to the following links.
https://www.careers360.com/colleges/sri-sivasubramaniya-nadar-college-of-engineering-kalavakkam
https://engineering.careers360.com/articles/tnea-cutoff

Thanks!!