Est. Budget: $100.00
Important notice: hold it!
I had found an app that may be an acceptable solution for him.
It is now being tested at his school.
I will know in a week if it solves the problem.
.p.s. - This is not 100% what I wanted -
I wanted this functionality within any iPad apps.
But as far as I learned - it is impossible to modufy or install new keyboard.