The only reason for getting a dx is if it gets him tools that help him get by in the near term.
Look at all the above stories, and realize that it’s not necessarily that there is something wrong our kids- a LOT of it is that we are trying to make everybody dance to a single tune- and that tune is not a natural one for adolescents.
Many of these ‘problems’ improve dramatically, or even resolve, with old-fashioned maturing- and from moving into arenas where success is measured differently than a school classroom, where the needs of the school system, not the needs of developing young adults, are paramount. The things that measure success in high school are often very narrow and incremental- and are not the same as those that measure success for adults.
With time, he will get better at some of the organizing and forgetting things- but, he will probably never be very good at them. It will just matter less as he charts his adult path. One of my collegekids* has known since she was pretty young that she needs to get herself to jobs that come with PAs asap 
See if the two of you can team up to find ways to help him get through- and try and find places where he can be successful from time to time. It can be as small as working out that teacher X ‘gets’ him and that teacher Y gives fewer small daily assignments- and helping him get more of those teachers. Look for ways to work with hime (finding new checklist systems to help protect him from himself, etc), rather than you fighting with him to try to be something he isn’t. Knowing that YOU are on his side and see these HS hurdles as an arbitrary and tedious part of his world that have to be gotten through to get to the more fun part of college, not as things that are a meaningful judgement about him, can do wonders.
*who has left her suitcase behind at the gate not once, but twice. Btw, she thrived at a UK uni, where admissions is based on standardized tests (APs mostly) not GPAs, and where you only study the field you are interested in.