A friend of mine asked me to help him advise a high school senior who wants to be a designer. I've known and worked with a bunch of different designers over the years, as well as being personally interested in Design, so I have a pretty good clue. Here's my take:
Well, you are right, of course. But, still, the usual path for a designer is to go to art school, work in-house at an agency or in a business, and then do freelance, if they want. maybe open up their own agency.
However, as long as a designer can find clients, then they can work anywhere. It's just that it's harder to present proposals for campaigns if you are far away from the client.
There are also very important economic considerations. In order to support himself, he needs clients that have projects that are big enough to pay for the cost of the time it takes to land them. If his available local clients are people that want business cards for $60, and a flyer for their new location for $100, well, he needs dozens and dozens of those per year, in order to feed himself. So, in this situation, they need to walk through the door on their own, by the dozen, without much sales effort from him(maybe he set up a marketing campaign for himself, and offers a service that he can do quickly). Or he can land bigger clients that need entire campaigns: packaging, signs, magazine ads, store displays. Then he has to spend time landing them, but the payout is higher so he can afford to spend the time. That's how agencies work: big clients, big projects that require lots of different types of expertise, ability to meet strict project timelines.
Developing his skill:
Looking at this from a formal point of view, as in, a manner in which someone who went to an art school might think.
The role of a designer is to: Communicate, engage, inspire, stimulate.
He needs to be a good communicator. He needs to be able to communicate ideas and values to the viewer, and to clients.
His work needs to be engaging. People are attracted to it and want to look at it. It holds people's attention.
His work will be inspiring. People will want to take action based on what they learn from his work, how it makes them feel.
His work will be stimulating. It may stimulate the senses, or stimulate conversation.
If you look at stuff that ad agencies have created, from super bowl ads, to soup ads in Better Homes n Gardens, you can see how the designer attempted to address these goals.
So, I think that if he starts looking for these things, out in the media world, he'll start to develop an awareness of the ways that designers accomplish these goals. I feel like I learned a LOT from looking at magazines, especially ones like Graphis and Communication Arts. These days maybe Pinterest might be a place to find works to study. (The problem with the web is that there is just sooo much crap out there. Graphis and CA are good because they are curated, and have a limited amount of stuff per issue, so you get better stuff, and engage with it longer that just flipping from "pin" to "pin".)
Then it would be good if he could practice doing these things.
Does he have access to any kind of page layout program? Microsoft Publisher, Adobe InDesign, Serif PagePlus? Maybe he could work on doing some posters or flyers for school events, or local causes. There's a bunch of stuff I don't know anything about that relates to getting things printed on actual printing presses. He'd have to learn that stuff too. Pre-press. But for doing school posters or flyers, he could probably output to an inkjet or laser printer and then he would begin to understand how to place images, and get text to flow from one text box to another, and play with font combinations, and layout composition.
I hope this helps. I put some thought into it so it could have some actual valuable advice, at least for someone who wants to be a designer in the usual way we think of designers. But there is a whole world of possibilities out there and maybe your young man will think of something entirely new.