This is a never ending problem with no easy solution. In this circumstance, I'd let the kid attend but would note that he isn't guaranteed anything when he returns regarding playing time. Some other player is going to play in that game and that player deserves consideration.
Our family used to have a family reunion which was also my Grandfather's Birthday celebration. He raised all of his family since his parents died in a flood. I missed that reunion one time for a baseball game. My Grandfather died a couple of weeks later. Again, I'd let the young man go.