Hi @Madeleine_Marie and welcome to our forum!
This sounds like an amazing trip that you have planned there.
Santorini and Rhodes are both magnificent (for both reasons each) and they are 2 completely different islands to be honest.
Rhodes is a pretty big island though and you definitely need some means of transportation (I recommend renting a car if possible).
One thing that might be tricky for your trip is the connection between these 2 islands as it might not be that straight forward.
Have you figured that out or not yet?
You may have to go through Athens first and then take another flight (I do not recommend taking the ferry to Rhodes at all…)
If you definitely have to visit Santorini I would combine it with another island which might be easier to reach using Santorini as your base : Naxos, Paros, Mykonos or even beautiful Milos island.
He is definitely allowed to come over as the wine tours are much more than just wine tasting…Depending on the winery you will learn about the vineyards, the islands history and the history of wine production on Santorini which is exciting anyways. He will obviously not be allowed to taste any wine though…