All the posts aside from the first one have been combative. WHY?
As said we aren't mind readers, and ANOTHER thing to keep in mind is we don't know you. When you say you've "done all the basics" how do we know what that means and what that includes, people have very different ideas of that. I knew a place who rescued a mare I now own, they fed her up and she looked fine but they did nothing to address her ulcers or her very severely compromised teeth, but she LOOKED good, and I'm sure they would say she was in top condition. You also mentioned several things that are less then ideal (what you're feeding in particular) especially for a rehab project (alfalfa IS ideal for a rescue). Saddlebreds, as a breed, often struggle with poor toplines, NO ONE said that is what the problem is, or that there is NOT a medical issue, simply that he may ALSO have some breed specific issues going on, or that his conformation is enhancing a medical issue. No one is telling you what to do just making suggestions based off research and experience, which for lots of people on here IS significant. No one is saying their suggestions are the best, in fact most of the posts are asking for more information. You can't start at the end and not expect someone to ask about the beginning, I would also be concerned about the quality of the answers if they did just start blurting things out!
You also posted one small and (for me at least) sideways photo from an awkward angle showing yes, a bony spine, but also a bony hip *and rump* and a neck that doesn't look too filled out. Now you say that picture isn't even current. I know I asked for more pictures to try to get a more accurate understanding of what was going on.. If I showed you my elbow and told you I was fat with poor muscling you would probably laugh at how absurd it was to show you that picture and expect a response. Yes, completely different, but the lack of information is not.
I am glad you and your multiple vets have a diagnosis that you did not 24 hours ago. Personally I don't think it sounds at all like EPSM, which part of it is classic? I'd be curious to learn more about the similarities since I am not seeing them with the limited information given, nor how the treatment would be that different from what was suggested? I almost wonder if you mean something else. Would love to learn more, and maybe even to be corrected- it is something you should be able to get a definite diagnosis for and treat and that would be fantastic.
Please don't be rude towards people trying to help you with the lack of information you gave them to work with. No one assumes you're stupid, in fact I saw a bunch of people trying not to assume, surely that's ideal?..