obstruction?

[BenGilli]

I had a situation last year, where a runner advanced to second base on a passed ball. Once he reached second, the short stop, said, that was a foul ball, you gotta go back to first... the runner then looks at me, and I tell him to stay put, but he heads to first and is thrown out.. I didn't call anything but the out, not sure if i should have given him second base.. This was a teener league game.. let me know what you think. Thanks!!