I'm unsure how old your boss is but it could be because of the way he was brought up. Especially the baby boomer generation, I feel like majority of them are very hard working and don't see travel as part of something they want in their life. He has also spent a long time investing in you and seeing you grow, so he may just be very heartbroken that you will be gone.

Although, I do feel like a great boss would support you in what YOU want to do and you shouldn't let people stop your actions. Follow your dreams!

Regarding your return. Think about it when the time comes. Who knows what might happen during your trip. You might think you like your current field but actually come across something different (as you mentioned).

I hope the best for you and following to find out more about your trip :)