I do think he could be drafted now, but it would be relatively late, whereas with another big year he'd have a much higher draft slot.
I also think you're right that he could have a huge season at BYU. The only reason to transfer is if he never really particularly wanted to be at BYU, but that it ended up being the best option for a one-season detour for his mom/family issues. There's also the possibility that he doesn't love the overwhelming LDS nature of the school or the honor code limitations.
While I don't think that's a LIKELY case (that BYU was more an emergency pit stop than a desired destination), I still think it's more likely than being a high enough draft pick to justify leaving that early.
My default assumption would be that he's staying for another year and then exploring the NFL. But if this report is correct and he's planning to leave, then my guess would be a transfer.