He has static encephalopathy (alcohol exposed). In the simplest terms, static encephalopathy is permanent, unchanging brain damage. By "permanent, unchanging" they mean it won't get any better or worse over time.
This doesn't change much, except now we have a way of sharing it with necessary parties without necessarily bringing up the alcohol-related part. There's just so much stigma attached to that, and we believe that part of M's story (his mother's alcoholism) is his private history, and not ours to broadcast to the world. So now we can just share the static encephalopathy diagnosis, or explain that he has a brain abnormality without revealing any other personal details.
Other than that, it's exactly what we thought, and we just have to continue our adjustment to this new realization of our reality.