Where is he going to spend the next summer?

Where will he spend next summer?

+++Oleg […spend next summer]

Dmitry, your version would be used if, for example, the subject used to spend every summer with his grandfather, but the grandfather just died. 

Uly're not right! Don't confuse Present Simple with Future Simple.

👍🏼😂 You made my day 😜😅

Uly’re hahahahahahahaha

Dmitrii, don’t confuse yourself with the native speaker who’s just explained to you the difference between your version and Oleg’s. Funny how you’re trying to argue with someone who, by definition, knows English better than you.
Oh and this “Uly’re…” is just hilarious. Do you need help with understanding the English tenses and how they’re formed?

My dearest heart, Dmitry, maybe I should have explained this more clearly. The future with GOING TO is used when there’s a concrete plan. “He’s going to go to Oxford this summer.” The corresponding question, which expects a CONCRETE ANSWER, is “Where is he going to go to school this summer?”

However, the future WILL doesn’t have this certainty. In fact, in a question, it doesn’t assume that the listener has an answer, much less a concrete one. It’s almost rhetorical in this way. The speaker is more wondering than asking. For instance here: "Ronaldo seeks new club after United exit, but where will he go?” This headline doesn’t expect an answer. It’s basically engaging the reader/fan to wonder as well… I/We wonder where he’ll go. https://www.thescore.com/uefa/news/2491448

By the same token, if you ask “Where will he go this summer?” (without any context), the native listener will assume that there’s an underlying problem or impediment that leads us to wonder where in fact this person will end up this summer. For instance, if it’s a boy who used to spend all his summers in the country with his grandfather (or in Runglish: …in the countryside with his grandpa), at the grandfather’s funeral, the parents could ask themselves “Where will he go this summer?” in the sense “Oh no, where can we possibly send him this summer — his grandfather’s gone!”

I hope this makes sense. Let me know if I can explain it further.