First thing-Are you SURE this fish is Serrasalmus Rhombeus?
If yes, do you have any kind of current going through the tank? Rhoms come from faster flowing waters and deffinately benifit from having good water movement. Have it set up on a timer or have plenty of "dead spots" for the rhom to seek refuge.
Water changes are good per week. Probably want to look at a bigger tank as that might be one reason for the lack of growth.
When it comes to the food....one week without food is nothing to these fish.
How I have gone about this in the past is: Not offer any food for about a week and a half to two weeks. Then offer what I'm trying to get my fish to feed on whether it be tilapia, catfish, squid or shrimp (in appropriate sized pieces). I only let the food sit on the bottum of the tank for one to two hours max before I remove it. I offer the food ever night the same way for about 3 or 4 days and then starve him again for a week before offering again. Repeat the whole process.
Gonna be a challange. Your Rhom has been eating goldfish (which are very ****ty btw especially for growth) and will be very difficult to ween him from. But you can do it.
Personally, I don't think you should get rid of him....Yet anyways. 3 years is a long time to have a fish just to give up. I would change over a new leave completely...bigger tank, lots of current, good water quality and way better diet and I think you will be suprized.
I'm also a little suspicious about internal parasites transmitted from the feeders (which is an extremely good possibility.....probably wouldn't hurt at all when you cut out the feeders to treat with PraziPro.
Depending on where you are, Rhombeus' can be extremely difficult to obtain. I would do everything listed, give it another year and if theres no improvement then decide on what I was going to do with him.
Good luck with what ever you do.