I don't have any answers, really, for you but I do have sympathy.
My 27 year old mare just went from being relatively blind to nearly completely blind (from ERU as well) this last April. She still has some sight but all she can really see is very very blurry, dark, vague shapes using her peripheral vision.
The first couple of weeks were absolute torture. I could see that she was confused and hurting and there was really nothing I could do to help her besides just being there for her. I felt so hopeless seeing my friend like that.
Thankfully she's an "adjuster" and always has been. She always seems to make the best of whatever life throws at her. After a couple really hard weeks (where she lost a ton of weight and was terrible nervous about everything), she somehow picked herself back up and now she's content again.
I think in our case, we were lucky in that we already had a really strong horse+human herd bond (she lived basically alone, with two llamas that kept to themselves) so it wasn't as difficult as it might have been for her to adjust.
Anyway, for us, the things that seemed to really help her out were non-dominant buddies. When this first happened, she was pastured with 2 llamas who gladly allowed her into their little herd but didn't ever touch her. She got to boss them around which made her feel confident about things.
Now, I'm fostering a mare (sighted) for a rescue who's about the least dominant horse I have ever seen. She lets Lacey bully her, Lacey follows her wherever she goes, and the foster doesn't mind Lacey getting into her "personal bubble" constantly. They were insta-best friends and are constantly together. They even gallop around the field together, upon occasion.
It's really a win-win.
Anyway, before putting your guy down, consider pasturing him with a single extremely passive+tolerant horse.
According to all the literature I've read, blind horses usually do better with a single laid back "guide horse". Give a single horse a try, he might really like that. Did he have a best friend before he went totally blind? Try that friend, it'll probably work out great. :)
I would also start using words with him to describe his surroundings and start getting him moving outside the stall. You say he doesn't like to be led, I'd assume that's probably because he's nervous about his footing.
I use "careful" to make sure my mare knows the footing is unreliable and that she needs to take small steps, "step" to describe something she needs to step up to/step over/step down from, "hill" for a downgrade/upgrade, "stall" for her to find her stall and go there, etc. I can actually still ride my girl and she adores being ridden. It's an exercise in trust for both of us but we really enjoy it. There's something really special about cantering along on a horse you know can't see worth beans and who's trusting you implicitly to not let her fall.
If that all doesn't help to improve his life, like natisha what quoted, better a day too soon than a day too late.
With my girl, I'm using 50% as my gauge. If her days ever get to be more than 50% filled with pain or fear, I'm going to put her down. If she's sitting solidly at 75% happiness, I'll be planning for that 50% day. If she continues to stick around 90% happiness like she is now, she's going to be with us for some time.
When my mare first went as blind as she is, I thought I was for sure going to be having her put down sooner rather than later. Well, she laughed in the face of that and she's going stronger than I really thought possible.!
*hugs to you and your guy*