No way he drops that far in the first round, especially if he has a good combine.
I actually don't think it would be shocking if he did, he has some knucklehead in him and he and his handlers are in full scale damage control mode over comparisons to Johnny Manziel.
I'm not sure he can put on enough of a show at the combine to offset some of it, and people are going to start picking him apart a lot.
I have no idea why he decided not to attend the draft (maybe he just wants to watch with his family as some other kids have chosen to do in recent years) but it very well could be that he and his people see potential to drop out of the 1st rd, and he doesn't want that embarrassment.