You have two different issues going on here.
Students are part of the graduating cohort based on they year that they started 9th grade anywhere in the world. Your child is probably part of the junior cohort with an expected graduation date of June 2019.
However, based on credits accumulated, he may have not accumulated enough credits to meet the state’s requirement to be in the 11th grade Don’t be caught up on your son being in 10th grade, if he does not have to make the whole year of work up. He can make up the credits that the school feels he needs to meet for promotion requirements and still graduate on time.
Did the high school explain how they are evaluating the transcript (using just giving a grade of CR-credit if passed, or NC -if the student was not successful) and allocating credits?
If the student has already taken math and bio, you cannot get credit for the same course twice. you (the student’s guardian) needs to find out why the school will not accept the credit for a course that he has successfully passed. When he applies to college, he would submit all of his transcripts.
In NYC we use this as a guide for evaluating foreign transcripts
www.uft.org/files/attachments/evaluating-foreign-transcripts.pdf
Here are the graduation requirements for the state of Illinois
https://www.isbe.net/Documents/grad_require.pdf
also read through the student handbook
http://shs.d211.org/
You need to know what age must students be enrolled in school and if he meets the age where he can be withdrawn without child protective services being involved. I know in NYC, you cannot take the GED until you are 17.5 and you will have had to miss your cohort’s graduation date. In your son’s case, the earliest date would be July 1, 2019.
Breathe, because you need to weigh the pros and the cons of your decision. You have already stated that he cannot take the GED in Illinois until he is 17 years old. IF your son takes the GED, he may not be eligible for instate tuition which may be based on him graduating high school in your state (after attending 2 years). In addition he may not be eligible for merit money/scholarship at some schools if he has a GED vs gradating from high school with his cohort (you need to play the long game).
From the school’s perspective they should want to work with your family, because your son not graduating with his class will be a drag on their graduation rate. I would definitely have his grandparents meet face to face with his counselor and your son to see what a graduation plan would look like for your son to be able to graduate with his cohort.