No restrictions from myself !
At 18 he is an adult
He can marry, fight for his country, travel the world !
I would recommend
Investigate the post Brexit rules for the maximum 180 days for staying in European countries.
Get passport
Buy travel insurance
Investigate what he will be charged to use mobile phone data abroad. He can ask his mobile phone provider.
Get a Revolute card or credit card that charges zero to use it abroad or to take cash out of ATMs
I recommend visit Edinburgh as a city or Liverpool or London
Also suggest
Fly to Denmark Copenhagen
Or
Fly to Schipol Netherlands /Amsterdam or ferry
Train s straight to city centre
Book accommodation before going
You can buy an "I am Amsterdam card" which gives cheap publlic transport & reduced price entry to some museums.
Walk round or find walking tours.
Food & drink is expensive, but there are supermarkets
I also recommend Iceland, expensive but worth visiting & he can join group tours
Turkey is much cheaper, because not in Europe
Look at Youth Hostels
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When I was young
I travelled for a month round Europe on the train
I slept overnight on the train & saved paying for accommodation between cities
He should Investigate travel via trains
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Suggest that he joins "Extreme Dat Trips" group on Facebook for plenty of ideas * he can meet people
Sky scanner to look at cheap flights, book direct with each airline (not via a 3rd party)