Carolina - Depression is one of the known serious side effects of drugs like Adderall. My son tried a similar drug for ADD when he was 14 - and became severely depressed within 4 months, we had to take him off of it.
Since this behavior change has all occurred since he started the drug, I would be very, very suspicious that the drug is causing it.
As for the "unknown neurological disease", if it's unknown, how do they know the genetics and the age of onset etc.? Do any other illnesses run in his family - things like celiac disease, diabetes, thyroid problems, anemia, etc.?
You might also want to check out info on the depression fallout website. And remember to try not to take it too personally, I know from experience it's hard to live with the depressed spouse sometimes because they tend to blame you for everything. (He didn't play football in school, btw, did he? Did he ever have a serious concussion?)
