This nearly happened to our boy when he was traveling in Europe. He was visiting friends in Germany. They decided to spend a weekend in London and flew from Bonn to London. On arrival the friends went to the EU line, but our son went to the non-EU line. He was 16, his first international trip and had traveled from the US via AMS without difficulty. He was met at FRA by his host family and there were no issues.
Having been separated from the host family by the EU/nonEU lines in London, the British customs ask him what he was doing in London. He didn't know the itinerary, and told the customs guy so. He was asked if he had traveled alone, and being the dufus he sometimes can be, told customs yes, thinking he meant from the states to Europe. So, being clueless, completely honest, he was told that he was being denied entry and would be sent back to Bonn on the next airplane available. So, he asked if he could talk to his host family waiting on the other side of the exit from where he was. Lights came on, questions were answered, and our now better seasoned traveler was admitted to the UK.
Don't know if Argentina might have been that way, but it sounds like your son is a bit more seasoned and travel smart than ours.