Hey, Lady. . . from everything you've said, I don't think this has anything to do with his eyesight. While it is true that these frogs have very poor eyesight, it's never stopped any of them from (eventually) finding food. And if you've been hand-feeding him OR using a syringe/baster/eyedropper, there is NO doubt that he knows where it is! In my experience, frogs will stumble around and snatch at the substrate while in search of food, but they smell it in the water and they DO find it. . . The variety of foods that you're feeding him is wonderful, and - aside from beefheart and black-worm - a lot like what I feed my own. . . Brine, Mysis, and Bloodworm are fine, but anything much bigger than that does need to be chopped into tiny little bite sizes for them to handle very well - but it sounds like you've got that under control, too. . .
Is it possible you've been OVER feeding him? You've only had him for a month, and it took me a bit of time to figure out that these guys really don't NEED too much food. I don't know how old your little man is, but an adult ADF only really needs to be fed every other or even every third day. I know that when I feed my frogs by hand, I tend to give them a bit more than I usually would, and if you've found leftovers on the substrate, it seems like *maybe* he's getting more than he can eat? If it were me, I'd try fasting him for a day or two, let his system clear out, and see what happens when he's a bit hungrier. . .
Refusal to eat *can* be a symptom for many illnesses, but it sounds like he's behaving normally otherwise. . . keep an eye out, though. If you notice any other unusual behavior, start looking into the possibility of illness.
The only other thing that I can think of is temperature. . . and water parameters, in general. If the temps dip too low in their tank, they are unable to digest food properly, and it can cause many issues - these are tropical frogs, does he have a heater in his tank?
You said the water is fine, and that you recently did a 100% water change - is this an uncycled tank? Huge water changes *can* stress a frog out, especially if he's been 'cupped' or removed from his habitat while you did it, perhaps he's feeling a little bit spooked? Do you have a water test kit? Toxin levels in a small tank can rise very quickly, and these frogs are sensitive little critters when it comes to ammonia and cycling issues. . .
I hope you figure out what his problem is! I really don't think that this is a vision problem, there is something else going on here. . . I'd really try skipping a meal or two and see what happens next feeding. . . good luck!