NIT

He can try for the branches other than CSE and ECE in low range. You can likely get into some IIITs but not the top one's. The specific IIITs available will depend on the category branch and the overall seat availability.

With a JEE Main score of 96.6, securing a seat in the top NITs for Computer Science (CS), Data Science, or AI is hard. In NITs like NIT Srinagar, NIT Jalandhar, NIT Bhopal, and NIT Patna might be possibilities of lower Branch like civil, chemical, if you have a good rank within your category of ST SC you'll get higher branch like computer branch.

Your NTA score is 20.8 means your estimate rank is above 5 Lacs, so it's very unlikely for you to get admission in NIT, until you are from ST category, but you can go for a private college that accepts on the basis of 12th class or management quota.

Yes, Mechatronics is good in VIT Vellore as it involves a modern curriculum, strong labs and research, good faculty, and industry exposure. Mechatromics is equally good, I'm IIT, NIT with slight differences.

With a 73 percentile in the JEE Main and belonging to the ST category, admission to NITs for B.Arch is unlikely for top-tier NITs, but there's a chance for some NITs and other good architecture colleges. You should explore other colleges and consider participating in the JoSAA/CSAB counseling process.