Whenever you join a course, there is a minimum period and maximum period during which you should complete the course. The registration is only valid for that period of time. For example, B.Sc Computers is a 3 year course and the maximum period of time that you can complete the course is in 7 years and that is the reason in 2017 your brother was able to appear for the examination for 3 subjects. But now it is 2020 which means it has been 13 years that he registered with the University for this course and so his registration must have expired. So he needs to register afresh. But ideally we would suggest you to talk to the College to check the procedure to see if he can complete the remaining 3 subjects or has to registered afresh and complete his Bachelor's degree.