Asking him would get a more currently accurate answer, but the parts I do know include that he had the knowledge and so wrote a self-published book (paper and digital) about GameMaker some years back, and he is good at knowing about and being a positive part of his gaming communities, so he has built his name recognition over time, initially while working a full-time job and going to school. And I think toward his customers ("students"), he treats them with respect, and they say good things about him. I think he has a designated # of hours per week he spends on things like those. Plus, his personality and strengths line up well with the kind of work he does: he likes art, and gaming, and talking about them, and helping people, and has a sort of natural sense about people and a speech cadence that work well in instructional videos.
Other things that have helped him stay in business are that he and his wife both wanted and agreed early on to take Dave Ramsey's advice (which somewhat they also got from their parents' example), that it is worth being very frugal to prioritize debt avoidance, having some savings, and paying off a house, over many other kinds of expenses, so when there are ups and downs, they can ride them out.
Edit: They drove that old car for a long time even though the A/C didn't work right and the windows wouldn't all roll down properly etc. Then they saved up for a while, watched for good buys, and replaced it with a reliable used van for cash.
Edits: They also pray and pay tithing, and there are blessings promised in Malachi 3 (Bible) and elsewhere to those who do that (properly?), which I have also experienced, not in identical ways. And they are family, so my wife and I pray for them all, often.
Plus he sets objectives and works regularly on weekdays, and takes breaks on evenings/weekends to enjoy the family. And they read business books and discuss them.
How does did he initially get customers? How does he keep growing?