can my 15-year-old brother leave the UK to return to his home country without being accompanied by the person accompanied by his British visa?

my youngest brother, who is 15 years old, is going to visit me in the UK with my dad as an adult accompanying him. But my father leaves for only two weeks and my brother will return there two weeks later. They come from Indonesia.

My question is: will they let him get on the plane to go back to Indonesia? Since there will be no outgoing immigration from the UK, I presume that there will be no problem?

I will send him to the airport until he gets on the plane. Please, give your opinion. Thank you