...except they're not really settling, and certainly aren't the 'confident' cats they were described as, 3 weeks on. (Got them vaccinated at the start of the week - vet said they weren't confident cats and probably shouldn't be in a home with young kids)
The girl still runs and hides whenever you walk in the room. Sometimes tolerates attention from us, but appears fearful most of the time. Until you switch the lights off at night when she turns into a lunatic racing around the room and letting out all of the energy she's pent up from hiding all day.
The boy is better- he will play with our son when he's in the mood, for a few minutes, and he will accept fusses from all of us. But he almost never initiates it, almost never comes to us, and still starts/spooks at sharp noises, doors opening/creaking, and sudden movements by us. He also shows no real interest in leaving the bedroom where we've kept them up to now - tried a handful of times and he keeps running back for cover in the bedroom if something spooks him. He's better with my wife and son than he is with me, but we're talking degrees - he's not really a social cat.
So...what do we do now?
- the girl clearly needs a different, quieter home. It's not good for her here. Rescue centre agrees...they acknowledge they've made a mistake.
- the boy is more difficult. He's absolutely beautiful (genuinely amazing markings, nice face), he does like a fuss when he's in the mood, and he's very gentle - no claws, teeth or malice in him at all. But we're still going to him, he still seems nervous at least half the time...not (yet?) really the confident family cat we were looking for. And properly distrustful of our 3y.o. daughter (understandably so...

I guess my question is as per the title - if we keep him and work on him, will he change? / how MUCH will he change?
I keep reading different things on this topic. It appears that they weren't really socialised for the first 5-6 months of their life (home with multiple cats owned by two very old people who probably wanted company not pets to play with, and as new kittens they were bottom of the pecking order), but can that human socialisation now be learned, or is it a lot harder as they're past 6 months?
I feel like I need to make a decision soon as my son is starting to bond with him, and the closer we get to Christmas the more difficult any separation is going to be for both of them...