I have a very complicated adult child - disabilities and mental health causing issues that make it unbelievably surprising that he is alive, he hasn’t irrevocably harmed me, on the path to success and we are involved with each other. We have a contract that addresses virtually everything and it is a zero tolerance contract in that if he screws up, we withdraw all support. It is surprisingly successful - probably because we once had to withdraw support and we stood our ground.

I have another adult child with special needs though pretty uncomplicated - low IQ, but decent job and understands and accepts his limitations. For him, when we need to have a serious life issue conversation (not like who are you dating but how are you paying your bills), I text and wait a few hours before calling. Trying to talk without warning is a disaster.

All in all though they are adults that have free will. I just want to help them be successful at adulting.

Moving kids with special needs towards adulthood and independence is so hard sometimes.

I suggest giving him heads up (an hour or so) before you engage with him. “I need your help with x in an hour”, “i would like to go for a walk in an hour and discuss x and y with you”. Bring up your incremental linear steps from a>b>. Get feedback for a>b before moving on to c.

Although in my case they may be reversed?
Only child who also was difficult to raise. He needed special ed services but there was an endless battle with the school district for eligibility--they initiated first, then when a teacher was denying services a couple of years later and I called them on it, they said he was a perfectly average kid (who nonetheless spent 4 weeks in first inpatient and then partial hospitalization). Formal diagnoses over time include major depression and other flavors of depression, ADHD, generalized anxiety disorder (they never really grasped the OCD side of that), Tourette's and psychotic disorder at the time of the aforementioned hospitalization (he was 10 at the time). He clearly ended up with PTSD and self-diagnosed at 18 when he read an article about it, the school stuff was tip of iceberg for his difficulties growing up. That battle continued--sometimes yes, sometimes no, pretty much useless when we did get a yes, he dropped out and back in and back out. At 18 he voiced suicidal ideation but was unable to get him hospitalized. He did get a GED with extremely good scores and he became self-employed. Drugs (weed, psychedelics) and a lot of drinking during his young adult years. Nothing I could do about it. Then, after a third (!) DUI he dived hard into his work, ankle bracelet to monitor for alcohol use for a year (major load off my mind). By 31 he had a serious girlfriend and decided it was time to buy a house.

The girlfriend moved to the other side of the country. He was diagnosed with a serious auto-immune disease could not do the work he had done. He had other unrelated serious medical problems. His house turned out to have undisclosed serious problems discovered when he had some remodeling done and has turned into a major money pit and new problems keep emerging. His business still exists but mostly handled by other people (including myself). He saw a couple of therapists, one of which did give him a medical PTSD diagnosis.

Given the things he's been facing, I had to become involved. He has said at times he could not have survived the last couple of years without my help.
He's always struggled with emotions and rage attacks--self-directed in a large sense. He regarded his achievements with his business essentially as revenge for what schools and other services never provided. He has never blamed me for his childhood and teenage struggles. His father is deceased (another struggle, happened when he was about to turn 13). He's in this cycle of pushing himself to accomplish something (with the help of cocaine because his prescribed medications aren't enough to combat brain fog and fatigue due to his diagnosed conditions), then withdrawing into sleep and TV binge-watching, then hopelessness about his life and anger/rage at the universe and the world. When he's not using the cocaine he does have a difficult time getting much of anything done--fatigue, attention and memory problems helping things go wrong (misplaced car keys, frantically searching house, turns out they're in his pocket).

The part I want to figure out is the communication. Given that he is very smart but also has the ADHD and in general a completely different way of thinking and also responding to things. It's like he has a universe of thoughts rolled into a single sentence. I am EXTREMELY linear and deal with problems by breaking them down and being very linear, which I can also get stuck in. Many many times conversations derail because we are speaking--literally--different languages.

I accept (as much as I can) that his problems are his and not mine (but there's a lot of grieving going on because I saw him reach this peak and then. . . ) but he DOES need support and I'd like to understand how to make that support effective. (It's still appreciated, I know that and he says that very often, that doesn't mean it's effective--and I have a deep worry that he needs supports who are NOT me for the day when I not there to provide that).

(I hope the person who have the truly thoughtful responses to the previous OP can do the same for me)