Depending on what is more important to you, the fish or the filter, could you cut the plastic above and below him and make a gap for him to get out? The longer it stays in there struggling to free itself, the worse the damage is being done. I had a fish nearly decapitate itself after getting its head stuck in the hole of a plastic breeder box. In the end I used pliers and a hammer to smash the breeder box and free the fish.

It's a pretty horrific death for a fish. I personally couldn't let any of my fish suffer like that. It may be better just to take the filter and fish out of the tank and work out a method of humane euthanasia. You could probably use the ice water method if you had a container large enough for the bottom of the filter to fit into. Otherwise, it may sound unpleasant, but blunt force trauma is the quickest way to euthanase a fish. I have done so in emergency situations. I wrap the fish up in a wet tissue or paper towel so it doesn't struggle or get stressed, and then hit it hard on the head with a hammer. The fish is dead before it realises it. It looks like perhaps the fish has enough of its head sticking out that this would be possible.

 

Hmm.. going to try a couple more things, but if he can't get out then, I'll most likely have the BF put him down when he gets home here shortly... poor guy. I'm pretty positive he was already dying before he got stuck so, probably just best to put him out of his misery. Thanks for your advice!