yes, maybe go to school to ask teacher who teach the kid (if your kid say no and sure you trust him) cause the school teach should know very well the situation
you have to prepare the worst case if really your son sing this type of song, doesnt matter, kids learn very fast maybe he just pick up the words from other kids...... what you need to do is to support him and give him positive guidelines (dont blame him) ...
many people likes to complain, so .... who care? Just make sure your kid know what is good for him and what is bad for him is fine....