IIT

Yes, one year is sufficient for IIT preparation.

At this time, Resonance is best for IIT preparation.

In my opinion resonance is best for IIT preparation for freshers as well as repeaters. Best of luck.

Kotak is the best in term of IIT preparation. If you spending money then Kotak is the best option as it has a better atmosphere for study and competition.

For JEE Mains preparation, I suggest you Resonance.

Prepare on your own. Don't rely on any coaching institute, instead get a good internet connection and start studying online. This will be the best way to study and get the most out of your time. What you need is to focus your class 12th books Basics. NCERT textbooks are the best for IIT preparation.

Click on the link for more details:-https://www.shiksha.com/b-tech/ranking/top-engineering-colleges-in-india-accepting-jee-mains/44-2-0-0-6244

JEE Mains and you have to secure above 200 marks to get good NITs or IIITs. You have to take PCM in 12th and must clear the same with at least 75% to appear for IIT JEE. If you score less than 75% then you cant appear for JEE Mains. So, also reappear for 12th to appear for JEE Mains.

You say him to join the counseling process by JoSAA as rank is good, then she will sure get Good IITs. Please check the college closing rank for different IITs:
http://josaa.nic.in/Result2016/result/OpeningClosingRank.aspx

To get admission through JAC, you just have to score the minimum percentage required in these colleges, I. e, 70%. Moreover, you need to focus more on the Mains paper.