That's actually a good point. South Carolina and potentially Nevada could be pretty favorable for him. Maybe New Hampshire? I don't know if him being from the Northeast has much bearing there.
If I was Trump or the RNC, I'd fear Booker and Bullock the most.
56% of Dem Primary voters in SC were Black (and similar in most Southern States)... which would most likely provide a really big advantage for Booker.
(Not that his appeal is limited to minorities. Stanford, Yale Law, Rhodes Scholar, Good Name recognition, & good speaking skills ... will get a serious look from most Dem voters)