Allenbatrachus grunniens, perhaps? The grunting toadfish. From what I see in your photo.
I'm reading people do keep them in freshwater, in the estuaries where they live they do get the choice of going back and forth between fresh and salt and brackish.. The thing about keeping it in full fresh I believe is that there may be some micro nutrients needed in the fish's organism found in salt water.
Someone on monster fish keepers has had one in full fresh for two years successfully. However, with this sort of thing it's the difference between a fish living 5 years and 10 years. Things like lack of micronutrients wouldn't really be evident when the fish dies, so you never know.
I'm assuming yours is in full fresh? Curious as to how long you've had him.