Does the consent travel form need specific flights?

Hi, in july and august me(19),my younger brother(17) and a friend of mine(23) will be traveling central Asia. Mainly Kazakhstan, Kyrgyzstan and Uzbekistan. Because he is a minor he needs an consent form approved and written by my parents. Because we want to be flexible we don’t want to buy returning flights yet. But are unsure if we have to for my little brother. When you search it on google sources say multiple different things- some say you just need an itinerary with an estimate some say specific flights. Can somebody give more insight pls. Live in the EU- Netherlands for more info.