Other than letting him out of the safe room too soon, which you've already recognized as an error, you are doing all the right things. Kittens (not just Savannahs) all have different personalities - some pop out of their carrier and immediately adopt you as their new best friend, others are more cautious and overwhelmed by the move so need more time and patience.
I think if you can get Dexter back into the safe room you need to continue what you are doing - sit on the floor with him, read aloud to him, play with him using a wand toy. Don't leave food in the safe room for him to graze on, but bring it in, give him time to eat it, then remove it after a period of time, e.g. an hour - this way he learns that you are the provider of his food, which will make you more important to him. Try finding his favorite treat (freeze dried chicken is very popular with kitties) and see if that will encourage him to come to you. I'm convinced he will eventually come around, but it will be in his own time frame, so patience is paramount.
I think if you can get Dexter back into the safe room you need to continue what you are doing - sit on the floor with him, read aloud to him, play with him using a wand toy. Don't leave food in the safe room for him to graze on, but bring it in, give him time to eat it, then remove it after a period of time, e.g. an hour - this way he learns that you are the provider of his food, which will make you more important to him. Try finding his favorite treat (freeze dried chicken is very popular with kitties) and see if that will encourage him to come to you. I'm convinced he will eventually come around, but it will be in his own time frame, so patience is paramount.