pl suggest preparatory books foudation of iit for my son studying 7 class

Hello
It’s good that you are clear in your aim and are all set to give it an early start. A focussed and meticulous planning will surely help your son reach the goal. Your aim should be on strengthening the conceptual clarity in Mathematics, Physics, Chemistry and Reasoning.
You can also make him join some coaching classes as they help in building good foundation.
As far as books are concerned, you can refer IIT Foundation Olympiad Explorer which is available for both mathematics and science. Besides this, tell him to think logically and once he is done with the NCERT, he can also go through some of the reference books like R.D.Sharma so as to widen his horizons. moreover, you can get some notes from some coaching institute also. Hope this helps!!

Holy faith and BMA takent search too.