JEE Main

I don't have any idea about IPU.
If your son has 112 marks in JEE having general category, then he might not get any good NITs easily.
He can go for other private colleges.

You can try!
As per 1st counseeling list, 50 score can get you in your 3rd-4th preferred branch. So, yes. You can try and get admission. Visit: www.thapar.edu.

Yes! Undoubtedly. Last year, at the end admissions were made open. Means, if a student hasn't filled thapar form, he/she can get admission in vacant branches. So, yes. Ofcourse. You can take admission. You don't need to wait for last round. You will get before that as per first counselling list. Visit www.thapar.edu for more information. Regards.

The best NITs for Electrical and Communication Engineering are the National Institute of Technology at Durgapur, Allahabad, Jamshedpur and Warangal.

With this rank , the best you can get Mechanical or EEE in the NIT's other than top four that is VSNIT , MVNIT , MANIT , NIT - Jamshedpur , NIT - Surathkal , NIT durgapur . Though , you have a good rank , I would suggest you to chose better college with better stream , that is focus on college and stream both equally . Best of luck

Since you have great percentage in 12th, I would suggest you to drop as you don't deserve an average college. Moreover, work hard for it throughout the year and believe that you can make it to that.
Hence, take a drop and work for it.

Last year, the least marks required to get into mechanical were 101 in JEE Mains.
Hence, explore different opportunities.

Since you'll not get a government college with this rank, you can apply for a good private university such as LPU, KIIT, VIT, ICFAI, UPES, Amity, Sharda, Galgotia University, Manipal (recommended), Amrita.
Or get into one of the private colleges nearby you if you feel the placements are good.
Moreover, the last option left with you is a drop.