B.Arch

Mention your category here. If you are an SC or ST, you can get seats in both. If you are from open category, it would be difficult in both.

You need to attend for jee mains for barch examination paper.

Yes, definitely every column need to fill before appearing for admission.

Actually, in Sastra University, they won't consider B.Arch marks in admission process. Basically, for getting into Sastra University, there are two processes.
Stream 1: which is JEE + inter marks
Stream 2: only inter marks.
For your son, considering the given data, getting seat is difficult. There is no donation seats. But, some seats are available in NRI quota and NRI sponsored quota.

Please refer https://www.shiksha.com/careers/architect-13.

Yes, NATA or JEE Mains paper-2 is the compulsory exam for B.Arch.

Bachelor of Architecture (B.Arch) is a 5 years undergraduate programme, which can be pursued after 10+2 with minimum 50% marks with the compulsory subject of mathematics. Candidates can take admission in this course through the NATA/JEE Main 2 entrance test.

Go through the article below for details:
https://www.shiksha.com/architecture-planning/articles/top-architecture-colleges-in-india-mhrd-blogId-15727.

The final AIR will be declared after April 2019 considering the best score out of the two JEE Main attempts for all the applicants, hence based on current percentile we cannot predict whether you will qualify for NIT Rourkela or not. Wait for the final AIR declaration
Details:
https://www.shiksha.com/b-tech/articles/how-to-calculate-jee-main-rank-from-percentile-blogId-19953.