In my experience with non-feeders, when they get to the point of being triangular (back bone sticking out, skinny), and espeically being lethargic and showing a wrinkling of the skin (esp. further back on the tail), you are generally going to lose them. It's horrible, but it's an unfortunate fact of life. If you want to have any hope of saving them, you have to get enough nutrition into them to help them grow, not just stagnate. If I'm going to force-feed, I bite the bullet and feed them every 5 to 7 days minimum.
I stay on the lower end of size, but I try to keep constant nutrition going in. Something like a young mouse or rat limb, piece of a day old chick thigh, tiny pinky or pinky head or I tube them with rat puree as described above. (Nasty, have a dedicated blender for that mess.) Tubing is a little less stressful (depending on your forcing technique, it may be a lot less stressful), but you may not have all the equipment you need to do it.
Now, this being said, if your little one is as far along as you describe, understand that the stress of just forcing or tubing him alone may push him over the edge, but you really have no choice (other than to euthanize him). When I get one to the point of skin wrinkles or acting lethargic, my general practice is to euthanize them. I'm trying to remember a single one at that point I've saved, but I can't come up with one, unfortunately. I'm not trying to be harsh or say that it's impossible to save them, but I'm just trying to be realistic to let you know what to expect.
I have one little gal I'm fighting with at present who ate like a monster while I had her, then refused all but 2 meals with her new owner. I have her back, and despite trying every trick in the book, we're down to force-feeding her pinkies. She's maintaining and seems to be bulking up just a bit on the feeding schedule, but I'm having to force her a day-old pink every 5 days. She's active, and physically was thin, but now looks like a normal hatchling, but she's not out of the woods. I am contemplating a short brumation for her to see if I can jump start her appetite. I keep checking her for kinks as I've seen them develop them when they suddenly quit eating like she did, but so far, she seems to be in perfect health, with perfect stools, just no appetite. It's so sad when they just seem committed to starvation and nothing you can do will change their mind.
