All filters need to be cleaned occasionally but with Oscars, because they should be producing more waste than your average fish, that occasionally should be more frequently. What can happen in a filter is the floss or sponge section can get covered causing the water to not flow through them but either around them or only through a smaller open section of the material. The downside of canister filters is that you can't see inside them when they are running.

I just checked my filters at least weekly or more often depending on what I found inside regarding water flow.
Wild Oscars live in low pH waters in South America but farmed ones have been adapted to much higher and harder waters. Case in point, we have Oscars in Florida's canals that more than likely were hobbyist releases or farm escapees and which have very hard and high alkaline pH ( 8.6-8.8 ) water and are reproducing like crazy. Farmed Oscars are very adaptable.
As for your feeding regimen, it's hard to say if it is enough without seeing a picture of your fish to see body weight and the fullness of it's body structure. What you described with the fish not eating much of other foods definitely sounds like what you want to avoid, the fish getting stuck on only 1 food. Here's the thing tho, Oscars are one of the most intelligent fish species we keep in tanks and because of that, your fish may be manipulating you.

That OR, because of the high nitrates, fish suffer organ damage if kept in them too long so that may be what's happening now OR the instability of the water is just making him not feel well

OR since the fish is severely stunted, ( he should have been 11" at 2 years old and even bigger at 4 years old), it's all catching up with him. There's no real way to know which it is without ruling things out one at a time.
My first action would be to look into a larger tank or pool with more room for him to swim around without bumping into the sides. After that, doing weekly water changes to keep the nitrate level low with checking your filter at least once a week and cleaning as necessary. If none of these get the fish eating more or eating more kinds of food, the last OR is probably the issue and there's not much more you can do.