While roached back is a common feature of dwarfism, this is looking a lot like a total lack of muscling along the spine. The spine looks a lot like the shoulder blades sticking out - no nutrition, body has been feeding off itself, no muscle in the neck, back, sides, quarters, loins, forearms or gaskin. His back not being straight may be due to him being slightly stronger (more so before he was starved) on one side. As the deterioration progressed, the stronger side curved the spine. If this is the case, you will be able to straighten it with time - which will improve the waddle.
I agree that you should check the bite. If it is straight, your battle just moved to your side. If not straight, have a vet look at it - it too may be crooked because of muscle loss. Or the teeth may have become worn unevenly, adding to his problems. If they are simply unevenly worn, vet can likely fix. Otherwise, apart from the jaw having been broken, you may be battling dwarfism.
Really, when the horses are this far down, the best thing you can do is clip - it's the only way to get rid of and stay rid of lice, without constant bathing, which you really don't want to face him with at this point. Clipped, you can see sores, return of lice, lumps and bumps. Plus! you will be able to see skin improvement. Really "down" horses often suffer from apparent dehydration; their organs aren't working as well as they should. Your vet might want to do a blood panel, which while not cheap, may be cheaper in the long run. It will show you what sorts of things will help the horse, rather than experimenting. From the looks of his coat, I would guess Red Cell might help, possibly iodine and maybe folic acid. But I am not a vet, so talk to one before trying any of it. Dosing minis is tricky at best, as you likely know.
Best parts: all the photos, he's head down eating, you are keeping him warm, i will assume he has water always available. I frequently add a malt liquor to my skinny horses' food, but I am not sure how much I would give a mini. The beer tends to work because it relaxes various internal workings, allowing them to work better, and the malt is a natural weight gain food.
Good luck. While I bless your efforts, I am more inclined to hope there is a very special place for the person responsible for bringing any animal to this condition.
I rescued this little guy from the horse market. He is skin and bones and very weak.
I had to clip his coat as he was full with lice.
I am wandering if he is a dwarf. I find it a bit difficult to see as he is so skinny. He walkes like a duck and his back is not straight.
