“My son has just turned fifteen and is begging me, let him go on vacation with his friends. Argues, that their parents approve of their children's plans and do not put any obstacles on them. I am a bit afraid of this first independent trip.”
First, I would advise you to be careful in believing your son's arguments like: “Other parents…”. It doesn't have to be entirely true. If I were you, I would first of all contact them, after that, to find out, what their position really is. It would also be good, if you met all participants of a possible winter expedition.
Even if the results of your treatments were calming, I would think again. Admittedly early independence (especially in boys) has its advantages, but we must be aware, that today's times pose many dangers for young people. Your decision should also be influenced by the fact, how much trust do you have in your son and do you believe in his sense. Do you know, to what extent is it susceptible to peer influence and pressure? During his winter trip, he can meet people from outside his closest friends' circle. You do not know these people and most likely you will not have the opportunity to meet them, In any case, I would not allow myself to go away, until I found out, that the son is aware of all possible dangers, he takes them seriously and knows, how could he avoid them.
It is worth knowing, that young people, who have not yet 16 years and wander without adult supervision, must have written permission from their parents for such escapades. If they do not have such a document, and will be IDed by the police, may be suspected of running away from home. I think, that it is worth using this argument in a conversation with your son and persuading him, to put away on his own, winter trip for next year.