WBJEE

Yes, you will definitely get admission in Jadavpur University with 648 rank under SC category.
So, just wait for the counseling to start.
All the best for future.

Yes, he will definitely get admission in Government College.
The available college for him are:
Kalyani Government Engineering College
Jalpaiguri Government Engineering College

You are eligible for admission in various government and private colleges in West Bengal like:
Kalyani Government College
Jalpaiguri Government Engineering College
Heritage Institute of Technology
Haldia Institute of Technology
University Institute of Technology, Burdwan

WBJEEM (Entrance for medical) won't be held this year. All admissions to medical colleges will be done through only one exam - NEET. If you haven't applied for AIPMT, don't worry. You will be allowed to apply for the NEET-phase II exam.
Best of Luck.

Yes, you can get admission as the minimum criteria for the NATA is 80 marks. So, you seem to have a bright chance which will give you good scope.
All the best.

Chances are less for getting admission in Netaji Subhash Institute of Technology for mechanical engineering. But still apply for it.

Yes, private colleges under WBUT accept JEE main score.You can try Colleges like Heritage Kolkata, BC Roy Durgapur,Techno India salt lake etc.I will suggest you to try JAC Chandigarh counselling .You will get mechanical engineering at UIET, Punjab University which is better than lower NITs.

Hello Simran,
You may use the college predictors given below for the colleges that you may get with your rank or score:
1) https://www.shiksha.com/b-tech/resources/comedk-college-predictor
2) https://www.shiksha.com/b-tech/resources/wbjee-college-predictor

Hello Vishal,
You may refer the link below that gives the college predictor for WBJEE:
https://www.shiksha.com/b-tech/resources/wbjee-college-predictor

Hello Salman,
You may refer the link below gives you the college predictor for WBJEE 2017:
https://www.shiksha.com/b-tech/resources/wbjee-college-predictor