marjasse,
I took the liberty of having a quick look at your other posts to get an idea whether the 1500 you wrote is 2 part or 3 part. I assume it is 3 part given that your son has mild dyslexia and ADHD and that he has used extended time accomodation twice this year on in-school tests.
Given that, I think it is time for you to take a step back and weigh cost/reward. The cost of your son applying is perhaps 25 hours per Academy or ROTC program, each. Another cost is that of disappointment in failing to gain an Appointment/ROTC scholarship and what it might do to self esteem. Finally, there is the opportunity cost in that those 25 hours per Academy/ROTC program could have been spent developing some other area of possible future employment or service to country.
with a 1500 3 part SAT, and a history of recent ADHD/Dyslexia and accomodation in school, I believe your son's chances are less than 1%. True, he will certainly gain nothing by not applying, but there is also a real cost as measured in a cost/reward review. Do you want him putting in, say 100 hours pursuing two Academies and two ROTC programs with under 1% chance of a positive result? Would those 100 hours result in a better likely return in some other endeaver in his life? An example would be interning at an Architecure firm with those 100 hours. Like all HS Juniors, he is probably stressed to the max and not getting as much sleep as he really should have. Is it wise to add the time/stress of the Academies and ROTCs application processes on top of that, with an extremely small chance of success?
Regarding the ADHD/Dyslexia, those are completely different disqualifying conditions. He would need to attain a waiver for each, separately. I recommend you research this in the DODMERB section of this website. To my knowlege, he would need to show 18 months of no treatment, no drugs, no accomodation for either condition, and show that without treatment/drugs/accomodation, he is able to be toward the top of his class and toward the top of the SAT/ACT, without any accomodations.
Regarding his desire to be a Marine -- I can't think of any area of our military, save driving a plane or Special Forces, where an error in reading an Order would more likely result in lost lives. Do you want your son to ever have to face the possibility of his Dyslexia or ADHD causing him to be distracted, or misread an order, and as a result Marines lives were lost?
Oh, and in response to your initial question/concern about Tier 3... there is no such issue with NROTC - Marine Option. Marine Option has no Tier1/Tier2 targets at all.