I'd start off small - like playing games together which require turn-taking. Perhaps get a fun timer that he can set himself when his turn on the computer starts and let him know when the timer goes off in 10 minutes (or however long you choose), it's someone else's turn.

If it's ever a sharing situation over food (like snacks etc), we usually have the "one divide the other choose" approach. So one splits the snacks onto two plates, and the other gets first choice.

And another easy thing to do is make sure the adults in his life always share - and point it out.... share the jobs, share the tv, share the books, share the last piece of chocolate etc.

and I've found consistency is always helpful too. So whatever tactics you decide to try, share them with Nan and Pop so they can do the same.