Need help in beaten up by unknowns

My name is Imran and I am staying in Bangalore. Two days before, one incident occurred to my younger brother( 17 years old). He lives in Delhi with one of my friend and that day my friend went to his home to celebrate holi festival. So my brother was alone that night. So matter starts around 3:30 A.M in the morning, three unknown boys come to his room and started to knocking the door suddenly and so for 3-4 minutes, he waited and then he opened the door. As he opened the door, they started to slap him and snatch his all money and mobile. So as he told me that he recognized a boy from them and one more thing is they threatened him by saying that we are yadavs and will kill you. Around two years before, I also lived in that building on rent and identified boy is also lived in that building who is yadav. Yesterday, my bro went on to that room again in day and he confirmed his identification. So I need some suggestions what should I do now as he only recognized them and we don't have any proofs except this. Should I case an FIR against them ?? And if I do this, then which type of proofs and legal actions I should take further.