Why are you on the 'net asking the visa questions? Your son is 19 and born in the 1990s and evidently capable of traveling on his own, which means he can use a computer and find the visa issues out from authoritative websites that provide details of his visa requirements and stay limitations.

Of course, nobody knows where your son is from so who knows the visa requirements? Canada, Australia, New Zealand, the US and other countries all have different visa relationships with European countries and the UK has a different set of visa schedules than the Schengen zone.

"How are the youth hostels?" -- anywhere from horrid to fine.

"He will be a single traveler, are there any red flags I should be aware of?" -- Tons, but they all depend on where he's going.

For a lot of non-visa questions, tell the boy to go on the Thorn Tree Forum on Lonely Planet - it caters to a younger crowd.

You have every right to be concerned about your son's safety. Your questions suggest a lack of experience on your part, and those of us who enjoy helping others will do our best to help you, not berate you.

Going to Europe is not like it was in the "old days", when you could stay until your money ran out. Greece is part of the Schengen Zone, and there are specific limitations on visiting this area that might apply to your son. Since Europe is his target, and since most EU countries are part of the Schengen Zone, the above 90-day restriction will apply if you live in No. America. An extension is not possible. He is not permitted to stay longer than 90 days within the Schengen States, and must leave for another 90 days before he can return once his 90 days has elapsed. He won't be given a visa, just a stamp in his passport upon entry into his first Schengen country, and the clock starts ticking then and there.

If you live somewhere else you need to visit the websites of the countries he plans to visit to see what their requirements are for visas for citizens of your country.

I've never been asked to show a return ticket.

Hostels vary in quality but in general the ones in Europe are good. Of course it does depend on where he goes and which one he chooses, but he can access reviews on websites such as hostels.com, hostelworld.com or hostelz.com

The Rough Guides are good resources for young backpackers. He can also go to a library to look through the latest editions of whatever guidebooks they have.

Assuming your son holds a US passport, many of the answers can be found at http://travel.state.gov. Other countries have similar websites.