Yes, I read that in reports and saw the presser that was linked here in the thread. I was wondering if he was perhaps headed there but then realized there was an actual checkpoint or saw that there was border patrol or saw a sign that said "Checkpoint/have IDs ready" or something to that effect which made him turn around/make the illegal U-Turn.
It's also possible he had absolutely no plan and was simply driving. Did that car have navigation system or maps in the glove compartment? He did not have his phone on him, correct? (Thinking no Google maps with no phone)
ETA: Thoughts on the long drive straight through. At 15/16, it isn't nearly as hard to stay awake driving as it is in later years when you're older and the "excitement" of being a driver has worn off. I was able to make a few longer distance trips at around that age (16/17/18) with no sleep and I was not/am not a drug user. Add in the fact that he was unlicensed, grandma is buried in the backyard, dad is due back from his vacation, in gmas stolen vehicle and authorities or at the very least your parents are likely looking for you and not at all happy, the adrenaline alone could make staying up at least one night or more easily doable.