The 'Superman' in my game is more like Gladiator from Marvel comics in that his abilities are primarily Psionic in nature, with a limited amount of tech back-up. As such, his psi-abilities can be expanded on in different ways - adding both powers, special attack options and increasing the power level of the abilities as time goes on.
I also stipulated that his family line is descended (about six or seven generations) from a similarly powered hero, and the power level of the line has increased as time goes on.
But, most of this comes down to what kind of character you actually want to play. In my game it's not all about the power level. There is story content, mystery, new worlds to explore (my campaign is similar to a cross between Legion of SuperHeroes and Green Lantern Corps) and, of course, super villain challenges. The latest is a Parasite-like character who converts the energy he absorbs into specific powers - one being super strength (STR 16).
Does your brother have a specific 'niche' he wants to fill, or does he just want to be a Superman-type character? Do you have any specific background rules he needs to follow? Does he like a magical character (SHAZAM!) or more of a science character (Superman)? Is this a question of what powers? Or, are you looking for some kind of whole package superhero concept?
I'm partial to the Daxamite with a PowerRing, if he wants something a little over the top. Or, maybe a mystic that can summon the powers of ancient heroes such as Hercules, Thor, Jason or even someone folklore'ish like John Henry. With each new hero he 'discovers' he can forge a totem, or a tattoo, or whatever ... and be able to call on that hero's power (one at a time, of course).
You could go Modern Demi-God from a lost world, or even New God (ala Jack Kirby), my game is currently touching on the "Bards of New Genesis" - which are original ideas, not from the comics. He could easily create a Hunter from that world, seeking renegades who escaped from Apokolips and into the mortal world.
There are so many options, I think we'll need a little more to go on - unless one of the above ideas sounds good.
Improving the character without breaking the game is mostly about balance and story. You as the GM have to provide challenges that meet the power levels and abilities of the characters, while preserving the shared story you and your players are trying to create. The players need to understand the role playing aspect of the game, especially this game. It is written so broadly, with so many possiblities, it becomes very easy to disrupt if players don't respect the nature/background and themes set down for their characters in the game.
Initially, my players were all "blow the bad guys away" - until I set the premise that on this world, life is highly respected and that any death would be met with significant social sanction - even if it was justified. Even the GL rings still have the lethal force restrictions, as well as the yellow restrictions (I set the game a few thousand years in the past & in a different sector of space).
Another thing I've had to reiterate to the players is that their powers can easily kill people, even minions or villains, and that they need to decide what power level they will be using when they fight. There's also property damage and the political and social fallout of any actions. You can save the city as many times as you like, if you do it by smashing the church, day care center, and messing up the water & plumbing for three weeks - there will be problems.
So, both you and the players need to set reasonable controls. Balance characters with strengths and limitations that make sense. My "SuperBoy" character just discovered a few weaknesses - namely a metal that ignores just about every energy except gravity (force fields, Telekinesis, heat, lightning, etc. do nothing to it), and a major bad girl made a few bullets - and put one in his head (he's immortal, so once the bullet was removed, he was fine - but it stopped him cold). The player loved it & also discovered that he isn't immune to HypnoSpores either.
Guide your brother into making a hero that isn't invincible & that actually cares, not one who's just in it to punch the lights out of bad guys. Once the social conventions are solid in a game, it becomes much more interesting.