This question probably can best be answered by the school counselor. Many school districts have credit recovery programs for high school students who are behind in credits. The school is monitored and accountable for students completing high school in four years, so this may be an advantage for your son.
There are so many factors about not completing credits or passing the End-of-Course (or grade) exams. Consistency is very important and encouraging your son to build on his skills helps. Students cannot study for these exams because they represent an accumulation of knowledge on a specific subject. I would also suggest that your son build on his strengths- pass the courses he can master more easily. That will give him an incentive to continue and not be discouraged.