My 7 year old son plays competitive chess. He can checkmate with king and rook nicely. He can make a queen from a single pawn & king. Now, how to proceed for further endgame training?
When he is mating, is he just following a method blindly or does he understand it?
For instance in the two examples you mention, zugzwang and opposition are important themes, which are also relevant to many endgames. You can find many easy pawn only endgames with these motifs and since they are so fundamental, I'd start with these motifs. Somewhat related, triangulation, key/corresponding squares could also go here. All of these you can do in pawn endgames which, because of the few options you have, are generally easier to understand than if you add any other pieces.
Some more specific topics could be king and rook-pawn vs king (draw/stalemate), mating with two bishops (which is not all that difficult if you understand that two bishops can build something like a "wall" that the enemy king cannot cross and your own king can relatively easily push the enemy king to the border), square rule to quickly assess whether a king can catch a pawn before promotion.
If you think that these are too complicated at his current level, I'd forget about endgames for the time being and focus on opening/middlegame play.
I highly suggest the book 'Complete endgame course' by Jeremy Silman. It starts from the most basic of all mates starting with 2 queens and continues for more general endgames like Lucena and also touches advanced endgames including analysis of specific grandmaster games.
Along with the book, I suggest setting up positions to test if whoever is being thought understands what is happening and can do it easily. For example, drawing a endgame where the opponent has a pawn but you have the (distant) opposition, where the opponent is a computer.
Endgame in general requires lots of practice and experience. Do not expect one to be amazing at endgames after 1 to 2 years of practice.
Another way of practicing endgames after understanding the concept is to play Guess The Move for endgames by people like Fisher or Capablanca.
I personally feel learning endgames is better than learning middlegame after one does not blunder in the middlegame. From experience, players rated less than 1700 to 1900 do not study advanced endgames, so if one is able to survive the middlegame, knowing how to play a complex endgame is incredibly helpful. This doesn't mean to play openings like English or French which are less tactical, playing openings like Sicilian or Kings pawn/ Kings gambit can train tactical skills when learning. Of course when in tournaments you would play the openings which are more silent.
"chess is a game you learn backwards"_G.M. Susan Polgar. One of her basic lessons is to put 2 to 4 pieces on a board vs a lone K and ask how many #s can you make ? Single book with probably the most variety "chess"_Lazlo Polgar..you need to edit for a 7 yr old- over 5 thousand problems ."how to beat your dad at chess" fairly recent # only book by G.M. Murray Chandler [ solid book with ? title] .Susan Polgar, Dan Heisman, Bruce Pandolfini are all modern style authors who write for novice players. In my opinion many of the mainstay classics are a bit dense and hard for kids to understand. Suggest ..stress Control the opponent vs just seeing the + . And of course keep it fun..use mistakes to get better..value the experience ..SLOW DOWN all newer players move to fast