Mahindra Ecole Centrale

With 23000 rank, you can get a seat moreover in some newer NITs or your home state NIT. You may or may not get CSE ECE IT but you can get other branches for sure. Why to give donation when you are having a decent rank. You can try for LNMIIT, IIIT ( international one's) which gives admission solely on the basis of marks and no rank. And regarding fees. LNMIIT is a bit costly which cost about 3 Lacs per year. IIIT may cost about 2 Lacs per year. Hope it helps.

Yes, your son can definitely get a seat in CSE on the basis of Intermediate score. It is too good.
Best of luck for your future and if there's any query, feel free to ask.

Please refer to the link provided below for a detailed fee structure for B.Tech in Mechanical Engineering from Mahindra Ecole Centrale College of Engineering:
https://www.shiksha.com/b-tech/course/b-tech-in-mechanical-engineering-mahindra-ecole-centrale-college-of-engineering-bahadurpally-hyderabad-230047

Follow the links below to verify:
Rank prediction : https://www.shiksha.com/b-tech/resources/jee-main-rank-predictor
College prediction : https://www.shiksha.com/b-tech/resources/jee-mains-college-predictor

No, there is no chance but you can give SAT entrance exam conducted by them.

You can get colleges of your state May be government colleges. Try JEE college predictor to get more information and insight about college which you can get on the basis of your rank-
https://www.shiksha.com/b-tech/resources/jee-mains-college-predictor?utm_source=connect&utm_medium=CR_JEECP&utm_campaign=4067795.

No, it takes admission for B.Tech programme on the basis of IIT Main exam or SAT exam. The SAT exam is for UG admission and quiet tough and mainly in foreign colleges. Also, you can get the admission through class 12th marks. If you got more than 90% marks, it may help you for admission.