IIT

If you are sure that you can do better next year or you can get some good college or a better rank in JEE, then you have to wait for it, otherwise don't waste your time and money.

Yes, if he got above 75% in PCM and admission on the basis of JEE Mains performance depends on the category to which he belongs.

Follow the link below:
https://www.resonance.ac.in/answer-key-solutions/JEE-Main/2017/jee-main-2017-eligibility-predictor.php

If you do Mechanical Engineering, you will get B.Tech Mechanical Engineering degree. If you do Electrical Engineering, you will get B.Tech Electrical Engineering degree.

For engineering courses, JEE is the only exam to get into IITs.

Drop is not necessary step for IITs. Many IITians haven't taken any step. If your focus on your goal is clear then you can also clear IIT JEE without drop.
Your concepts must be clear whether you take a drop or not.

You may use following link to find colleges where you can get B.Tech CSE or IT : https://www.resonance.ac.in/reso/news/JEE-Main-Branch-Predictor.aspx.
You may share few choices out of link recommendations for further advice.

I think you can get top NITs with this rank.

In which state are you looking for?