Bruno tries to help Shmuel find his father despite being frightened and wanting to go home. Why?

Shmuel reminds him that he promised he'd help look for his father. Bruno feels as if he must keep his promise.

Bruno had promised shmuel they would look for papa and then they would play but the soldiers blew a whistle and they had To go in a Camp and Bruno thought it was for keeping them from getting wet

Bruno opted to helping Shmuel despite his fear partly through loyalty and he felt obligated to keep his promise, because of how he had denied his friend earlier on, and he wanted to make himself worthy of Shmuel's friendship as he knew the importance of putting his friends before himself, even in the face of danger. He also did it because he knew that he was once again being made to leave his friends and move, and he wanted just one last adventure. Once they were inside the camp, with Bruno in the spare pyjamas, the whistle is blown, and not everything goes to plan, although the boys don’t understand exactly what is going on, rather misunderstanding the procedure for staying out of the rain.