Parenting is a tough job and it sounds as though your son is adding to the difficulty, perhaps due to his diagnoses. When he is in school, is he attentive in class and does he participate if given the opportunity? Perhaps he could do something else to show he is learning besides his homework.
We applaud the fact that you have removed his electronics and we hope that you continue to do that until he meets your expectations and does his homework completely and consistently. What does he do when he should be doing his homework? Does he use other privileges that he enjoys? Do you have him doing chores around the house? Does he still have to sit at the dinner table for the amount of time it would typically take for him to complete homework?
If there are other children in the family, do they have a structured homework time when there are no distractions (no TV) and it is quiet? Do they do their homework in their rooms or in a common area of the home, such as at the dinner table? (Your son should not be allowed to seclude himself in his room because that too would be a privilege.)
Have you met with school counselors to see if they have any ideas or options other than allowing him to fail? Check with them and try good ideas they have that would be helpful.
You are right. You cannot force him to do his homework. You can only motivate him with consequences and control your response to his noncompliance.
If your son has a strong interest in anything, perhaps you can figure out how to tie that in as a consequence to doing his math assignments or his reading or writing assignments. Be creative in making those connections and perhaps you will be able to spark his interests.
Good luck and let us know how we can help you further.
