B.Arch

Hello,
No, it's not compulsory to take JEE Mains Paper 2. If you want to pursue the B.Arch & B. planning courses then you can appear in JEE Mains Paper 2. JEE Mains Paper 2 exam is conducted for admission in B.Arch & B. planning courses at popular Centrally funded Technical institutes (CFTIs) and National Institute of Technology (NITs).
Best of luck!

You can easily get a seat in there.
For more information, refer the link:
https://www.shiksha.com/college/chandigarh-college-of-architecture-3568/admission

Yes, you are able to get a government college, as you have got 245 rank in JEE Mains paper.

Admission is through counseling conducted by Directorate of Technical Education, Maharashtra based on marks obtained in 10+2/ Diploma and score obtained in National Aptitude Test in Architecture(NATA).

For NIT college, you have to score around 200 marks in JEE Mains paper. But, for IIT you will have to qualify JEE Advanced exam.

Yes, you can apply for Chandigarh College. 95 percentile in JEE Mains is a good score and class 12th marks are just meant to be qualifying criteria for student.

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.