Chances are, the coach has no idea your son wants to play another position -- especially is he's playing his current position well.
Your son should pull the coach(es) aside before or after practice and discuss his concerns. Make sure he doesn't have his mind set on only one position he'd like to try, because that will be difficult for a coach to work with. If he suggests a few, then I think once they're aware, the coaches will probably start rearranging things, at least in practice.
After they've heard it from your son, if no effort is made to change and no reasonable explanation is offered, then you should speak to them personally.

Have seen this before and Chelsea is right about your son talking with the coaches. I know our coaching staff "ignores" parents input because it is invariably skewed. However, if a player expresses an interest in another position we entertain the option depending upon the skills we see in practice. At the least we discuss the options, either pointing out skills to work on (which we don't have time to do in practice) or, as has happened, explain what he is delivering in his current position. We let the linemen know that THEY are the key and without the blocking and the holes we don't have a chance. So, if he is really good our staff would be pointing that out and praising him for his work.