How to lift a 13 year old boy up from medicrity to say 1600?

SCENARIO: A grandson just turned 13.   Plenty of hometime on the PC with this Coronavirus social distancing rules.  Just joined another site to play some chess instead of shoot 'em up PC games.  He's only a beginner.  He can play, but knows nothing about strategy planning etc. 

At thirteen it's a wonderful opportunity to set him up for an enjoyable experience in chess.  

What How to think in chess / how to win at chess DVD would lift an eager lad out of the swamp up to say 1600?

LOTS of instructive resources and suggestions here for chess improvement....browse...

https://www.chess.com/blog/RussBell

Solve lots of puzzles. Chess.com has many puzzle resources such as Puzzle Rush, Puzzle Battle, Daily Puzzle and so forth.

Age is becoming less and less important in the chess world nowadays. If your child can think, then they can play chess and 13 is certainly old enough to understand the game - yet still young enough for the mind to collect new ideas like a sponge. I'd encourage them to create a chess.com account (or another website as you've done) and simply play some chess. If guided with resources like theoretical endgames, puzzles, pattern motifs explained and so on, then practice alone can do wonders.

If you or your grandson want to play some (unrated) live chess games on chess.com, with me, then I'd be happy to play and offer my opinions  

Ultimately it is up to them though. Chess takes a lot of dedication (as do most things in life to reach "success"), so that means that they must be motivated and there is little others can do for this. If your grandson doesn't want to play a lot of chess, or get "too" into it, then they simply won't improve much regardless of their resources. However, if chess is their idea too - then there is a long, yet rewarding, journey ahead

Practice with dumbbells, the more You pump the higher You will be able to lift him. 

All the really strong kids I've played/lost generally have had coaches, if you can afford one this I think is the best way

Players 7,726,770

Avg Rating 973.43

https://www.chess.com/leaderboard/live (Graph on the right.)

above mediocrity would be in the region of 1000-1200.

1600 starts to get into the top 10% of players. 

A more realistic goal would be anything above 1000 ELO.

Let him just play and enjoy for the moment, and try otb as soon as possible.

She probably means FIDE/USCF which, based on statistical posts I’ve seen, tend be higher than chess dot com’s ratings until about 1800-2000 when they fall in line and eventually behind.

That said, I really noticed my game improve after reviewing computer analysis of my games, especially my losses, and after getting diamond membership to unlock unlimited tactic puzzles.

Have them play! Preferably g15 or longer. Work on tactics. This site has tactics, other sites have tactics. 

Let's this be "fun" 

If they develop a passion then the rating points will come  

The best way for any beginner to improve is by regularly solving tactics puzzles.

If you spend 1 hour each day solving puzzles (taking your time for each one) you will improve a lot in the 1000 - 1500 range.

It would probably be good to get him some books on chess, especially and endgame book and a game collection (like Garry Kasparov on my great predecessors). Maybe an opening book or to slightly study and opening to make sure he doesn't get wiped off the board by crap players who memorise 100 traps although opening study shouldn't take up the most of his time. Being solid at tactics is also of paramount importance and there are lots of resources online for this area of the game. 

Above all, make sure he enjoys it, chess is a hobby not a job.