You can't just let this drop, you just can't. You need to find out exactly what has gone on for the moment OP will pretty much have to let it drop. She will have to allow the school time to investigate what has actually happened.
The Head can't give out any more information, safeguarding does not allow this. Remember Safeguarding is there to protect the teachers too!
But OP can contact the head, email would be good, and say that she appreciates his call but that she is, naturally, now somewhat concerned and that being left in the dark is, at the least, nerve wracking. That she appreciates his need to keep a lid on things but that she would like to be appraised of what has actually happened as soon as possible, could he give her a time frame?
At 17 is is not unlikely that her DS will have some idea that something has happened, he may even have a good guess. But, as he is 17, it might be tricky getting him to say anything. Though OP should talk to him about it, to let him know exactly what the Head has said, mainly because what the Head said is ALL the information that is officially known. All else is gossip, or someone breaking safeguarding regs!
Good luck OP. I hope it is all resolved quickly.