Purpose of Journey in Schengen Visa application form

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.