My processing speed is pretty bad too and all I can muster in terms of advice or insight as someone who was wild as a teenager and did what I wanted - it looks to me like he doesnt know how to cope with things and reacts instantly. He is probably self medicating, he may be low in energy or motivation and not just lazy, it sounds to me like you have a lot on your hands - have you suggested finding someone who specialises in sensory processing issues for adult, like a counsellor and insisting that he is allowed to stay at home if he does this?

It sucks feeling like the failure in the family and he has probably got a complex and low self esteem and probably really frustrated that he cant just be like everyone else and just do the right thing at the right time. Do as much research as you can and learn what is going to help him as a person with SPD, for me the only person who stood out in my life was the person who was fair without any anger, there was a punishment (chores) and then everything was forgotten about and I knew they would always be calm and if I did something wrong I had to do chores. I feel that punishments should never take something away that someone loves but to add extra responsibility instead, it is a better teacher, more positive and not negative.