Not to speak on behalf of the FDS team, but the game is not indented for use by children under age 13. That’s not to say it is prohibited, just not encouraged.
I agree that if we have kids playing the game we shouldn’t have this type of language visible to them. I’m not big on all the complaints on here about inappropriate callsigns, but this is a case I agree should be taken into consideration seriously.
I would suggest your son play on solo mode until (if/when) some sort of language filter is added to the username and callsign fields. Regardless I hope he is still able to enjoy playing!