I already said duas will be addressed either in this world or in the after life or in paradise. If you think your brother didn’t get what he was looking for, making duas for 10 years, then it must be something he had done, some kind of sin in a previous life and he is paying his debts in this life. And of course with Allah or any gods, by showing devotion now he has made his future towards light either in this world or for his next life. For example, he might have been a very lazy person in previous life, who did noting good and neither believed in God. So in this life he was praying for duas. After 10 years and he still didn't get any duas answered, if he thinks so then he might start new search for his cure. That makes him take towards the light of knowledge. Thus Allah or God might have planned to make rid of his previous sin in this life in such way that he will be devoted to God and also gain self enlightenment.