It isn't really true that Savannahs only bond with their first owner. Like Paige said, it pertains more to F1s and F2s however, that can still be overcome. The more outgoing the cat is the more readily it will bond with someone new with time. Sometimes they "chose" their person however.
In the case of your F5, I think you will see result over time. I have two rescue dogs, one of which was likely a street dog that had to fight to survive. It has taken a few months for one to really settle in and change. Accepting the situation as it is, is the best approach. Don't expect him to change at all and be happy when he does. I am sure he will, but when we expect something to happen in our time frame, it can cause a tension in the situation which the cat feels.
He may not know what a toy is, but likely there is something that will tempt him so keep trying different things. Looking forward to updates on him.