Does anyone know how long he can stay here as a visitor???
Three months, but:
A) The person who said if he overstays his welcome by a day that he would be thrown in prison is a moron. He won't be thrown in prison. He'll be forced to leave the country if caught, but do people get caught? No, not really, especially not if they come from a European country, the majority of which are visit without visa.
B) Legally, he only needs to leave the U.S. for only a second to reset the time clock. So, if for example an immigrant lives in San Diego, all one needs to do is go to the border crossing, go into Mexico and turn right around, and the 90 days is reset.
C) If he is young and is willing to work for free as an intern to gain experience, just about anybody will hire interns under the table. Its technically not illegal to hire immigrant "apprentices." That doesn't do much to find him a job, though.
Long story short, though, is this: for a European to find a permanent job in the United States, typically requires they have an outstanding skill/talent (actor, sports star, scientist, doctor, professor, PhD in economics) or that they recieve their undergraduate or post-graduate degree in the United States, which makes a company that sponsors visa far more likely to sponsor a visa.
There are temporary avenues. Depending on his age, Disney is one of them, offering temporary, visa-sponsored employment to young foreigners at Disney World and
Disneyland:
https://www.disneyinternationalprograms.com/