Son finishing Semester Abroad in Brussels in May we plan to meet him in Dubrovnik. Any ideas of best mode of transportation to get him from Brussels to Dubrovnik??
Airplanes. Cheap and fast compared to any other method.
You have to fly. Using anything else would take at least two days.
To find flights, use Skyscanner. If you can't get suitable flights to Dubrovnik, you could look into flights to Split; but, Dubrovnik has no train access, and is 4-5 hours by bus from Split.
Flying into Dubrovnik is pretty expensive in general. Financially it would probably be cheapest for him to find a flight on a budget airline to Split and and take the bus from there. The buses are comfortable and affordable.