@Coleoptera
Regarding your son, what genre does he write in?
Generally, unless your other son knows his way around photoshop and can create something which is essentially on-par with a professional cover design, that is probably what is hindering sales.
When it comes to selling books there are two key things to keep in mind.
Passive Marketing (the most important)
Active Marketing (not as important as passive but still important).
Passive Marketing is the things that will account for the majority of your sales. They come down to - Cover, Title, Blurb, Keywords. In that order. If you can nail all four of these components, you will sell books.
Active Marketing (facebook ads, amazon ads, newsletters, social media engagement etc) are almost always a waste of time if you haven't got your passive marketing nailed.
If you'd like to PM me the links / pen name I'd be happy to take a look and see if there is anything I can help with.
My question though is purely commercial: in your opinion and experience, which fiction genres are going to be selling best in the next couple of years?
I'd rather start by targeting a niche market immediately, that will be most likely to sell (obviously you need to be able to write but let's suppose I could do something decent enough). I know that romance, and thrillers always seem to do well but I'd like to focus on a very specific subgenre where self-publishing might be most likely to bring in an income. Any thoughts?
This is what I did when I started. I'm not snobby when it comes to writing, I'll write stories about whatever is selling and I write them extremely fast (50k novel written, self-edited, covered and published within 2 weeks).
Romance is the biggest selling indie genre.
Within that there are sub-genres, and then tropes.
A sub-genre would be paranormal, and a trope would be enemies to lovers.
There are two ways to get started quite quickly in romance and that is either "writing to market" or "writing to trend".
Writing to Market - You set out to create a product already knowing what readers want, and you do your best to give them exactly that. An example of this would be Western Romance. You'd probably have an alpha male, a woman maybe from a big city. He's tall, handsome and good with his hands with a wicked sense of humour. She's just been fired from her corporate job and moves to the sticks to look after her sick old auntie. She's a virgin, of course. The romance follows all of the beats (Read Gwen Hayes, Romancing the Beat), there is no cheating AT ALL, and it ends in a pregnancy.
You're giving readers exactly what they want and what they expect.
Writing to Trend - Harder to spot but if you can, there is lots of money to be made. Last year it was bully romance set in high schools / academies. Some misunderstanding causes the hero (usually an anti-hero) to bully the heroine. Girl fights back, hero falls in love, a whole lot of grovelling and redemption commences. Another popular trend was paranormal academy reverse harem. The thing with chasing trends is you need to be quick, and like writing to market you need to give readers what they want (sometimes without knowing what they actually want since the trend is still emerging. It's risky but if you do it correctly there is lots of money to be made.
If it's specifically romance you are interested in, then I suggest having a look around on Reddit r/eroticauthors
It's a sub for authors in Erotica and Romance genres. Check out the top posts. Emerge yourself in the community, and be prepared to spend a lot of time on Amazon checking out the charts, reading books, analysing the market, and reading reviews.
With all that said, sometimes the best way to do it is just to DO IT. This time last year I didn't know half the things I do now, and I still made more than my day job only 2 months in. As long as you're willing to work hard you really can learn as you go.
Hope that helps.