It sounds that he has used his position as a police officer to obtain the details, he needs reporting as this is a sackable offence , It sounds like he wants to frighten more than go ahead with any offence , He is acting like like victim, judge and jury , thats not allowed, he must be a right thicko falling in his own trap by investigating it himself , call his bluff and get this bully out of his job , we don't need coppers like that .
Don't be fobbed off by a sargent who will say he will sort it out on the quite, get it done officially and teach this bully a lesson .... Your son in law should also learn from this , let the dick heads race off and get in the hearse first .. don't rise to it , there is enough dick heads on the road .