The time to ask this question is at the end of the Fall or beginning/end of the Spring season not in the middle of the season.
Every situation is different due to the player, team, and coaching dynamics. I've observed it is wrong to presume coaches use logic in filling out their lineups. Some coaches put their lineup together based on stats. Some coaches play hunches. Some coaches play who's hot in practice. Some coaches play the kid who's dad is funding the AD or HC's salary. Some coaches put in experienced upperclassmen over inexperienced underclassmen. Some coaches play their favorites. Some coaches play to situations. The bottom line is you don't know what is going through their heads, and you can't control it.
Your son can control what he does and how he does it. If this really starts eating at him and affecting other aspects of his life (school work, think about quitting the team, etc) then maybe it is time to ask the coach what he needs to do to get more playing time. He needs to keep in mind that this has the potential to backfire on him. JMO.