It's like the ammonia poisoning has burned off all of his brain cells, the poor thing. There's no bloating so that's a good sign but he definitely looks uncomfortable/sick. To be honest, I'm not sure I can recommend you do anything more than what you're already doing. You've got him in all the right things, he's warm, he's comfy. Without more visible signs, it's difficult to say if he needs a medication or, if he does, which kind.
With the hanging at the surface behavior, it does seem like he could possibly have gill flukes. His gills seem a bit distended and the excess slime coat could be a sign he's doing his best to get rid of some foreign substance on his body. If you want, you can go ahead and add 1 tsp of AQ salt to that IAL mix and see if that doesn't help him a bit. With gill flukes, they don't always show the usual darting, itchy behavior that they do with something like ich. Instead, they seem to have great difficulty breathing, lethargy, clamping, excess mucous . . . pretty similar to his symptoms.
Some pet store bettas never show any signs of illness and yet they never perk up no matter what you do. It's almost like they gave up hope and gave up the will to fight so they waste away within a few days. :( It's sad to see so I really hope this beautiful guy isn't one of those cases. If he has any chance, it's definitely with you.
When I brought home my first rescue, Sapphire, he looked exactly like that..just no response at all, stuck in a corner, and hoplessly staring into space, and was like that for hours, only time will get him to adjust and a lot of coaxing..I talked to him a lot, and sat beside the tank for hours on end..watching and moving my finger to see if he would at least look at it, in the begining, he didn't but eventually started to look up, and around, and noticed my finger, and that was a good sign, then I tried to get him to eat,..and if you can get him to eat, and hopefully he will get some energy to start moving, unless there is more going on..but I know that sad look. it's soo depressing..I hope he comes around, the IAL and warm, clean water will eventually help, and good nutrishious food.
I'm so sorry, aokashi. The saddest thing of all is when all the tools we have to save a fish just aren't good enough :( At least someone cared about him at the end of his life, instead of dying along in the back room of a petstore.