I think he stops being Superman. This question is more "if a guy had the abilities of Superman...what would he do if he weren't a hero." See, that's part of Superman's cachet. All of us look at him and think about what we'd do if we had all those abilities. And we stand in awe that he's chosen not the practical or selfish thing, but the hero thing.
Had to say that. But, to answer the question. The most money he could likely make would be for NASA or some such. Near space exploration in this solar system. They could pay him a boatload, strap on a modified go pro, give him a little air and huzzah. The amount of money that goes into space stuff is huge. He would still be very wealthy, but it would save space exploration lots of money. He could be the cheapest way to build a base on the moon. Just have him haul a bunch of stuff up there and start construction.
Now, I wouldn't want to give him a job that can be accomplished by an ordinary person--thief, or, uh food delivery was on the list of the link.
Same thing with first response or bomb squad--he might do it better, but the job's not a unique one. I can't see the city paying him what a space program would.
There is something, that seems ordinary that he could do that would, in fact likely do something terrible to the world economy. Assembly line work. Basically he teams up with an entrepreneur/agent who figures out how fast Supes can work. He's stronger, faster and more precise than most machines and can get hours of work done in seconds. So his pay might be less than all those factory workers who would have done in a year what he accomplishes in a month--or less (his rate of speed and precision is pretty darn ridiculous). The agent would just have to pick the right project for max profitability.
Going to say no on drilling for oil (too many regulations, who would stop him, sure, but there will be fines) and to diamond making. He could crush coal into diamonds but...the diamond market is actually artificial. We have a lot of diamonds on this planet, it's just that most of them come from one place, and they create an artificial scarcity. I'm not going to go into the politics of it, however, making lots of diamonds would de-value diamonds. If he just makes one whopper of a diamond and sells it--that would be unique.
An agent would smartly diversify his portfolio of things he does, because, quite honestly, his skills could completely toppel economies, and turn public opinion against him. If I were his agent, he'd be saving people from burning buildings periodically, just to keep his image untarnished.
You talked about maximum giving for charities. Well, he could do one day of work for a specific charity. If he has celebrity status (which I think he would) he could bank on that. A lot of what he would do would be celebrity-type stuff.