I would love to believe it... :) . But could the vet be so wrong about his spine?
He didn't even examined his blood but was certain it is his spine and left me with the news that my horse is about to retire at the age of 4,5 :/
Yes, he could. It's a sad fact, but there are vets out there that have no idea about what they are doing, or worse yet, don't care. Does this vet know horses? I would think that his lack of muscle right now has more to do with the toe dragging than anything. You know the feeling of walking after running for a while, when your legs feel heavy and tired? I imagine that is similar to how he is feeling, because he doesn't have the muscling to sustain movement.
I'm a novice really when it comes to horse health, but all I can see is a horse weak from malnutrition. Feeding him carrots, apples etc is all well and good as a treat, but not as a staple part of their diet. I'm currently working on putting weight on my lease horse for winter, as he dropped some in the last few weeks. And I'm telling you, a week of hard feed (lucerne/alfalfa hay 24/7, pellets and chaff) has bulked him out immensely.
Right now, focus on getting your horse at its optimum weight. Heck, even get him fat! Once those ribs of his are covered by a nice layer, work on lunging to build his topline, and then get into riding. He's a beautiful horse, as is very obvious by the photos you've posted. He seemed to drop the weight rather quickly (unless his fluffy coat was hiding it), but you've already got him coming back nicely in that last photo. Keep it up, and you're going to have him back to an absolute stunner in no time.
Elana has given you some brilliant advice, as she always does! Definitely, definitely
show the vet those photos you posted of his progression.
But most importantly, don't give up hope. Even if the worst did happen and you couldn't ride him again, you would have an amazing companion. But the chances of that being the truth seem slim right now. Wait and see what the new vet says. Until then, act as though you're just fattening him up so that you can eventually ride him every day :)