Hi Kevin,
I think it is great that your son wants to travel with you and your wife. When I was younger, I traveled with my parents to a few places and had lots of fun!!! I have young, active parents, so it's easy to find common things to do together as well as some "me" time.
Anyway, from what I gather about the RS tours, which many love, it seems to be a good mix - ages, singles, couples, friends, etc. There is no guarantee of who would sign up for any given tour. (There was another thread with a similar question of solo traveling. The poster answered that she calls the RS office and inquires the "age range" and "single/coupled travelers" to get an idea.) Of course the office cannot divulge personal details but maybe can guide you to a level of comfort.
Find a country or two you all like and concentrate on that. I love Italy and Sicily, so I push for that, LOL. Look at each Italy or the Sicily tour, see if the itinerary is attractive to all of you and if it is doable. What is the focus? Art and History? Hiking? Beaches? Vineyards?
For example: Choose a couple tours, maybe print out the itineraries, each of you look at them and "compare" notes so to speak. Check off what would be attractive and important to you on this trip. Find a commonality you all can share but also have some independence of personal enjoyment. There is some free time on each tour for those that want to venture out for a couple of hours to do their own thing - extra sightseeing, shopping, people watching, wandering, etc. Keep in mind that some tours are more aggressive/strenuous for sightseeing. You can see that on the activity of each tour.
Also, maybe watch some of RS' tour videos of various countries. If you do not see them on PBS, you can access them through YouTube or on this site too - take a peek at the Explore Europe and the Watch, Read, Listen. Something will inspire you.
Time and again, it has been suggested traveling in the Springtime - May or June and then in September or October. As everyone knows, the height of the travel season is beginning soon - right through September, even October now, as countries and airlines have gotten smart.
If you love the hustle bustle of people and travel, then the summertime may be a good fit for your family. I hear June is quite lovely in most countries. July and August would be crazy busy in a lot of countries - August is usually the big travel month for Italians.
If you are planning it for this year, going now (Springtime) would need some fast planning. People do it. It depends. Some people can throw together a trip in no time; while others need a few months to get things in gear plus also taking into consideration work, securing home, etc. etc.
Good luck and have fun choosing a vacation tour. I am sure whichever one you choose, you will have fun :)