The thing about the -3/3=-1 and not knowing it is..he knew it, then he forgot. Then he relearned it and retained for a little while, and then forgot. It is as if learning new things confuses the old. Just before he forgot -3/3...we were working on slope intercept form...how to figure the slope...rise over run. I think it was learning that that made him go back and confuse the fraction of -3/3. As if he is trying to apply the new stuff to everything else when he goes back to everything else and it all mixes up. Sort of like...if you were making dinner and every time you add a new item, it somehow mixes with everything even though the marshmallows only belong on the sweet potatoes, they end up on everything. And each time you add the additional item, you have to go back and remake every single other item to sort it all out.
This is life with a math LD. This is my daughter. They genuinely cannot help it. She puts in tremendous effort. Huge. And every time we take an extended break (or even a couple of weeks off) she forgets a ton, muddles up a bunch and then struggles to get back on her feet. But the other component is that even in a moment where things may seem to make sense, they may not really make sense. Not in a mastery level sort of way. The connections and understanding may be very weak. They aren't sustainable when more data is added.
FWIW, I do not have the profound math issues that my daughter has. I still found Algebra very difficult. I took Algebra 1 in High School over a two year period with a teacher trained in teaching struggling students. He was wonderful and I loved being in his class. I did not retain the information. I took Algebra I again in college. Faster pace, not as good a teacher. I got a D and that was being generous. I didn't know what I was doing. I took it again at the college level when I was pregnant with DD. I studied HARD. I did a ton of extra work outside of class. Things finally made some sense. I did well in the class and enjoyed it. I did not retain it. Working with my own kids I realized that most of that information is just gone. I had to start over completely once again. If it weren't for the fact that my kids need my help I wouldn't even bother.
Honestly, though, I think I would have done better if I had been able to really solidify fractions/decimals/percents to mastery level before tackling pre-algebra and then done pre-algebra over at least a two year period, working really hard at review as we moved slowly forward with new material and never taking really long breaks. Working with manipulatives and real life application might have helped, too. It might have given my brain something to "hang my hat on".
My point, I guess, is that while this is exceedingly frustrating for the parent and the instructor (which can often be both) it is even more so for the person trying to learn this material. It is disheartening and demoralizing to work SO HARD, get a glimmer of understanding, and then it all disappears again. It can be so emotionally damaging to face that over and over and over. And the more emotionally damaged the student, the harder it is to think.
With DD, I had to walk away from any expectations of following a NT path/schedule for math. That isn't her reality. Instead, I started her over from the beginning, combined multiple methods/curriculum and tried not to push her beyond what she is capable of doing. I also worked hard to end on a positive note whenever possible, I have kept lessons short, I work hard to acknowledge that she is putting in tremendous effort even if the result may look less than stellar, and I have readjusted my reality to realize that milestones in math for DD may look vastly different than milestones for an NT kid. Nevertheless those milestones need to be acknowledged. I cheer her on. And I try hard never to define her worth or her future based on her math abilities.
I don't think there is going to be a magic bullet here, but there are ways to help your son find a good path to his future. Hopefully the suggestions and ideas presented will help. I wish you and your child the best.