How old is your son, and what grade is he in? Just guessing, but is he saying he doesn't want to continue with regular course work, and take a GED test and be done with school?
The GED test can be challenging, because it's a timed test, with just a few brief scheduled breaks. If he finds it difficult to stay seated and stay quiet and stay focused, he might have problems. There are 4 main areas of testing (math, science, social studies and language arts) so if he has problems with those, he might find the GED test intimidating.
Just knowing what's on the test is only half the battle. There are time constraints and costs (especially if you fail a part - you still have to pay).
Is there any way your son could enroll in a free online public school, like K12? My daughter had tremendous difficulties in school due to medical issues, and we took her out of public school in 8th grade. It took her 7 years to graduate from high school, but she did, 1 week before her 21st birthday, which is the cut-off. She took half course-loads, had tutors and a 504 plan for modifications due to her issues, and got that diploma. It was far easier and less stressful than trying to take the GED.
There are professional GED prep tutors and programs, which you can find online in your area. Maybe you could take him to one of those, and have him evaluated? They might be able to assess whether he's anywhere near prepared, or has the self-discipline to remain seated and quiet and focused for the duration of the test.