Until he crosses the Canadian border, he is still perfectly legal on US soil. The Border Patrol can stop him all they want on the US side, and he will be in legal possesion as a citizen. He could be "fair game' all they want, and he would not have broken any laws, unless they can charge with "intent to export"...
It's just a silly, ridiculous thought, and not really worth anyone arguing over, but it was running around in my head as a hypothetical, probably from 10 years of crossing the border on pretty much a daily basis...
To the OP, basically, unless a person could navigate the quagmire of quasi-legal, mission impossible, loopholes as described, the answer is NO. He may legally be in possession of it, but it would still be illegal for him to export it.