There are so many great threads and resources on here. Get little Ziggy in a quarantine room, grab a cup of tea, your iPad or laptop and get yourself in there, too. Hang out in there with him reading and reading and reading this site! Let him explore while you are just around not expecting a thing from him. Follow the advice of the experts here, I know it helped me immensely with socializing my own "special needs" f2. She has transformed into an (almost) normal Savannah. I never had a time table, I never expected her to be a snuggly cat, and I think letting go of expectations removed stress on me, which in turn removed stress on her.
Both of my f2s hiss as part of their "talking" and not because they are angry or being mean. They make a number of oddball sounds. I am curious as well to the dynamics of the other household too, as that agression of chasing seems odds. I wonder if ziggy hisses like mine do and the old owners took it as a sign of agression, perhaps punishing him? As for the child, again, did the child run with something fun to chase? Did the child chase the cat without being corrected? Kovu is not above stealing a stuffed toy from my 2.5 year old, but that is because as a cat he doesn't know those are Ry's toys, not his. Nyah stole fairy wings for the same reason, which resulted in a mad chase of 2 cats, a sobbing 2 year fairy with no wings, and a frantic daddy trying to get the wings while reinforcing our rule that you are not allowed to chase cats! Chaos, thy name is my house.
Freeze dried chicken is a fantastic treat. My cats love, love, love it and will now do tricks for it (sit and shake). I second that recommendation and again, can't stress enough how much there is in the way of good information on these kitties on this site. Good luck, and keep us posted on his progress. He is a gorgeous boy. I hope things all fall in to place for both of you as being loved by these amazing creatures is a very cool experience.