Son of a violent criminal = kid with no father at home, probably abandonment issues and probably an unstable home with a single parent who may be vulnerable.
All Hepworth knows until Catherine shows up in uniform is that Ryan doesn't have a mother or father on his school records. He may have heard rumours about his father, almost certainly knows his mother is dead because that's the sort of information that would be passed on to school staff. He sees him turn up to football with assorted adults, but not the same ones each time.
That is the impression of a very vulnerable and easily manipulated kid.
I'm not convinced knowing his gran is a police officer will necessarily put him off. It tells him his main carer is often out, has a high pressure job, so likely to be absent/distracted. Plus the job is likely to be a cause of some teenage embarrassment/rebellion etc. easy to persuade the kid not to tell her stuff etc. Also imagine how much more pleasure he will get from abusing Ryan when he is also getting one over the police woman who showed him up.
He'll think he's way cleverer than Catherine too because men like him always do!