Welcome!
As you may have seen in some of the other threads, it appears that taking the SAT for students under 13 is no longer an option without the cooperation of your local school district.
With regard to qualifying measures, do you have relatively recent evaluations related to his ASD and ADHD? How long ago did he take the WISC? If it was within the range allowable as listed on the DYS eligibility page, you might already have scores for that application. I think it's five years. And if he is not yet in grade 7, he should be able to take the PSAT8/9.
The question of college courses is distinct from DYS, as many of us have enrolled children in college coursework without that label. Check your state's dual enrollment/early college regulations for specific information on that. (Or contact your nearest community college or state university.) Where we are, the only documentation necessary for a homeschooled student to enroll in regular college courses is a letter from our home district confirming that the student is homeschooled, and placement testing administered by the college itself that establishes the appropriateness of the requested courses. (For a student your child's age, the admissions counselor typically would reserve the right to meet the student, as a general check on how they might feel in in-person classes with adults, but for my high-school-aged students, they didn't. This may differ in your region.) The SATs were actually not always sufficient for placing into courses around here, with the placement test (usually Accuplacer) the determining factor instead.
Depending on what his needs and interests are, sometimes even the remedial courses can have value for a student in his situation. For example, our CC system offers self-paced remedial math (high school level) courses that cover prealgebra, algebra I and algebra II, which potentially could be a convenient and relatively inexpensive way to cover those high school topics from a standard curriculum, but at a faster pace. Typically, students can take either more or less time to progress through the topics (so if you can finish all three in one semester, feel free!).
Other questions you might want to ask on DE/EC include whether students are restricted in time/mode of class (e.g., only online or only in-person, only after hours or only during school-day hours, # of courses per term), transferability of courses to other state or private universities (if that matters to you), and if there are discounts or fee waivers for dual enrollment students, including ones associated with specific demographics (e.g., first-gen college students, veterans or children of veterans, prospective STEM majors, students who were in the foster care system at some point, other historically underrepresented groups).
