Hm, that’s a journey anywhere. Assuming money is no object, you’re 8 hrs from hawaii, and 3.5-6 from florida depending on layovers and where. Europe is only 6-8 from you if you stay in western europe.
Having been to both of your choices many times, I’d suggest Kauai for its rugged beauty and hiking.
Have you considered taking him to jersey shore to remind him how good he has it at home? :)
Black rock or old airport, nā maiki’loa kahakai 🙂