Primary bilateral testicular agenesis is a very rare condition. What is possible in your brother's case is that his testes may still be somewhere hidden and producing male hormones. Now he is in puberty, if he has grown as a male with secondary sexual characters like male configuration, pubic hair, change in voice and growing of moustache then he has to have testes somewhere. You will better consult an endocrinologist to check his hormonal levels and make sure about the status of his source of male hormones. If he has normal male hormones then he will have normal sexual activity. However, if his testes are not in normal position till the age of 13 he will not be able to father a child.