My friend, it happens for a boy to have one testicle. He can be born with one. It can also go back in the abdomen and remain invisible because the path it passed thru didn’t close perfectly. But it can be removed due to having some problems of sickness or an accident. So when someone has a single healthy testicle, he can have children because it produces several sperms every day, and for both sex. So don’t worry for your brother, as someone with one testicle can have children unless he has other health problems, and this can happen to anyone.