I know this may sound silly, but have they considered giving him 'softer' shoes, i.e. shoes with more cushion?
I realize that may effect quickness at some level, but if it keeps him in the game, why not give it a try?
I recently had a hip replacement and walking was not terribly comfortable. Then my son got me a pair of Oofos sandals and OH MY GOODNESS...WHAT A DIFFERENCE. (BTW the tennis shoes not nearly as good.) I can walk without pain a much greater distance. He had read where Alex Smith, the old 49er/Redskin quarterback had said they made a huge difference. With me they did.
Perhaps some level of cushioning, an insert or something, might help David in articial turf games.