DnD was hard to learn when I gave up on it in 1979: I’m ignorant as to the current facts.
MASKS I ran with a family group (my neice and her husband, my nephew and his wife) over the Internet. It wasi interesting and I would recommend PbtA games as giving a great deal of support for the GM without the learning curve of higher simulation games.
I once ran a game of RISUS (using one of the published scenarios) to try to show one of my Wednesday night players’ neice how the hobby worked. I gave her my copy of the game as a gift along with a thumb drive with a selection of games from my hard drive.
I would agree that finding the genre or even the book/TV series/film/video game that sparks their interest. And the kindest thing that you can do is show them a game as it is played. Ensure that the group is on their best behaviour,though.
When I played Blades on the Dark I got a strong feeling of training wheels, forcing me into the approved style. To me as an experienced gamer that was annoying, but it mught be worth a look in this case to avoid the “so, what do I do” problem. (Also available for multiple genres.)
Having now played a bunch of Forged in the Dark RPGs (like Blades in the Dark) and also a bunch of Powered By the Apocalypse RPGs, I find the PbtA ones have game mechanics which are easier to understand, and are less keen to have you fail the majority of your dice rolls. FitD seems to be about things spiralling out of control.
Also the core conceit of FitD system game is that the players DO NOT PLAN the mission. It is supposed to be “we’ll sneak into the Sheriff of Nottingham’s castle at dawn” then straight to the action, not a 2 hour discussion about exactly how you’ll sneak in (disguise? distraction? cling to the underside of a cart?).
FitD games have crashed and burned with two of my groups, because they both contain players who adore planning and debating all the possible courses of action.
This has been my experience. There is no need to teach new players GURPS. Just have them describe the character they want to play, in English, French or Icelandic, as appropriate, and the GM converts that to GURPS mechanics. Then the player just roleplays their character, making decisions as if they really were this particular character in the events and situations the GM decribes. Actions which need mechanical resolution are resolved by the GM, though if they like to hear the dice rolling, players can totally do that themselves.
I’ve played with six-year-olds this way, other children and tweens, teenagers, other young folk, and even with people in their forties who had never heard of an RPG before, at least not ones which didn’t explode. To me, at least, game-mechanics are just the resolution system, not the point of the whole process. Players don’t need to see the task resolution system at work, any more than video gamers need to have a second screen open to examine the coding when playing the newest first-person shooter.
For the situation described by the OP, it sounds like the problem is lack of anyone to GM for the boy, with distance making it more complicated still. My instinct is to suggest a family member whom he trusts and feels comfortable around. If no family member knows GURPS well enough, you can probably GM without the player(s) knowing any rules with many other systems. As long as the range of possible outcomes and rough probabilities of the task resolution mechanics are close enough to pass muster with everyone in the audience (usually just the GM and player(s)) and there is no assumption clash, then any system will do.