Why do learning disabled children get the same high school diploma as other kids? He is a senior and is only doing the work level of a 7th grader. I understand him getting credit for time put in.

Please make your question littile more brief

If he was in the same classes doing the same work and has passible credits then he gets the same diploma. The problem is not making different diplomas for different credit ranges.
The problem is the pushing a seventh grade level kid through to the point he is graduating.
Was he ever given an IEP and placed in special classes? Why wasn't his abilities questioned and him held back or given special classes that could have caught him up?

Abrahambaby why would you ask for OP to make the question a little more brief when more details would be more helpful?