The only cause of an actual knot on the vas deferens is a surgical ligature for contraception. If your brother has an unexplained lump, he should see a doctor.
Most probably it is an inflammation called EPIDIDYMOORCHITIS caused by infection of vas. It can also be calcification or post operative fibrosis after vasectomy operation. If it is a bacterial or viral infection, it is very difficult to control because antibiotics take a very long time to reach vas. It can also be due to venereal microorganisms. Get semen examined.
Your brother could have an infection in the tube carrying sperm from the testicles out of the body or he may have been exposed to a sexually transmitted infection. Encourage him to visit a doctor and get professional advice because if it is an infection it should clear up with antibiotics