If you're using an analog thermometer for his tank, I'm betting your temps are actually colder than it's saying. Like Mentat said, kidney damage is entirely possible. 75 should be the low range for night-time temps. But with 75 as your avg/high temp you're probably looking at temps getting down in the 60's especially at night. Get a heating pad and place it on the side of the tank in addition to your heat lamp; also make sure the frog is in an area where AC can't blow on him (we keep the vents closed in our reptile room).

If a vet isn't an option (it's the best option, but I know sometimes a herp vet just isn't available), fix the temps immediately and do a soak in lukewarm treated water with several drops of honey mixed into it. This will cause them to pee and poop, eliminating a lot of retained toxins. Depending on the level of organ damage, doing soaks every other day for awhile may help out or it may not. Also, if you're having to assist-feed him anyway, try appropriately sized chunks of nightcrawler dusted with vitamin powder. They are more nutritious and easier to digest than superworms (stop feeding those altogether) and crickets.
I let someone else care for mine for awhile, and when I got her back she was having seizures and not eating; turns out they hadn't been misting, heating when cold, or keeping her cool when it was hot for quite a while. I was pretty sure she was going to die, but that was over two years ago, and she's completely back to normal now. Mainly, don't just assume the frog is going to die, because there is a chance he'll recover. But review what was said in that other thread; frogs are very sensitive, and ignoring correct husbandry can cause problems quickly.