Diploma

You need to fill in the admission form and you are in.

Yes, he can pursue B.Tech in CSE but as a fresher i.e. being a diploma holder your son is also eligible for direct second year entry (lateral entry) by clearing LEET examination but this will only be eligible for Electrical and Electronics stream.

B.Tech/BE through lateral entry (direct second year admission) will be best for you. you have to appear for LEET examination to get a good college.

Please refer to the link provided below for details regarding a course in M.Arch from Sushant School of Art and Architecture. You can download their brochure which contains important details like eligibility, application dates, fee structures and contact information.
https://www.shiksha.com/architecture-planning/interior-design/course/m-arch-in-interior-architecture-sushant-school-of-art-and-architecture-ansal-university-gurgaon-276305

Yes, you can. But only few diploma colleges offers Aeronautical engineering course.

No, there isn't. You cant do BE course in abroad after your diploma

Hello Aksh,
There is nothing to worry. You are eligible for JEE Mains if you have scored at least 75% in the Diploma you are attending. For more details about the JEE Mains ELigibility. visit:
https://scoop.eduncle.com/ugc-approved-universities
According to this, a student pursuing a diploma is also eligible for appearing in the JEE Mains exam, but only if you have scored at least 75% marks in your diploma.

You can but most of the reputed universities conducts admission for minimum 6.5 in IELTS.