I am travelling to Europe (4 countries) with my wife. During the trip, we shall be accompanied by our son who is already residing in Germany as a student. So, what should I mention as the purpose of journey in the application form ? Naturally, I thought of choosing Tourism. However, my question is whether I should choose Visiting family/friends as well since my son shall join us during some part of the travel. It is to be noted that, we are not going to be visiting his residence (in fact not even visting the city in which he is residing). As a follow up question to this, since the itinerary is independent from my son's plan, should I mention him in my cover letter ? Kindly advise me. Thank you.
Strictly speaking, your are not visiting your son, he is joining you for a part of your traveling.
Since the main purpose is traveling, even a last minute change of plans for a short visit would be acceptable (within the granted dates of the visa).
It should be added if, at time of application, the intention of a visit exists.