Colleges in Thanjavur

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.

Hi, you have a fair chance of getting into SASTRA. Do download the application form from the website and apply.
All the best!

Yes, it is possible in Sastra University to get admissions through Lateral Entry. For better information about courses you can go through this link https://www.shiksha.com/b-tech/course/b-tech-in-computer-science-and-engineering-lateral-entry-srinivasa-ramanujan-center-shanmugha-arts-science-technology-and-research-academy-thanjavur-300297

Yes, you can get admission in Sastra, but I'm not sure whether you can get your desired course.

Yes, you can apply for it. There are high chances of you getting a seat if you apply.

The fees for B.Tech in aerospace engineering is INR 3.96 Lakh.

Yeah, he had chances to get seat in bioinformatics or biotech in sliding can change to mechanical or EEE.

Yes, you can expect CSE there. Till 500, there's chance to get CSE.

You can get CSE there as you are a localite.

Yes, you can. In SASTRA, 70% seats for B.Tech course are filled on the basis of JEE Main score along with prior weightage to the marks in 10+2.
30% seats in B.Tech are filled on the basis of merit drawn on the basis of marks in 10+2 followed by counselling. In the case of admission through JEE Main over 70% seats, 75% weightage will be given to JEE Main score while 25% to the marks in 10+2. In the case of admission, on the basis of 10+2 merit over 30% seats, normalized marks of each board will be considered.
Best of luck.