I kept a male Argentea for many years, from the time he was around 4 inches, all the way up until he was about 13". Here's a few suggestions I can make based on my personal experience...
Firstly, you could try out some different lighting options, perhaps something that's not so bright. I used a single 24" fluorescent tube over the 125 that I kept mine in and he seemed to really appreciate that. Only about half the tank was lit up, while the other half was pretty dark. Personally, I really like that look and it provided him some darker areas to hang out in without the use of decorations.
Dithers can also help shy fish feel more comfortable. If you're looking for something that's more biotope-specific, Swordtails can do the trick. If that's not a concern, Congo Tetras, Giant Danios, or various Barbs will do the trick.
Some sand or other substrate may help him feel more at home as well. Mine loved to "play" in the sand that I used, moving it back and forth, creating piles as well as large pits. He would also pick up small rocks, take them to the very top of the tank, and spit them out against the glass so they make a "tink-tink-tink" sound all the way down the glass. This was easily his favorite thing to do when he wasn't eating.
As far as being a picky eater, it sounds like you're on the right track with the NLS. Frozen foods can also help. If you're able to find Lifeline Herbivore diet, try that. They also really enjoy snails. I used to bring home dozens of pesky trumpet snails and dump them in the tank for him, and he would go absolutely nuts for them.
Lastly, as you already know, he's going to need a bigger tank soon. Male Argentea can grow very large and really shouldn't be kept in anything less than a six-foot 125 as an adult. You've obviously got some time before he reaches max size, but it's definitely something you'll want to think about in the near future.