Yes your neighbour brother can sit again in Jee mains 2020. Jee mains can be given 3 times. Jee advanced can be given only twice. But i will suggest your neighbour brother not to give a drop again. Go for other government exams like wbjee, comedk. Or can take admission in private colleges. 

• You must be the citizen of India
• You must have passed 12th examinations with minimum 5 subjects and should have Physics and Chemistry as compulsory subjects
• You should have passed 12th in year 2018 or 2019 to appear for JEE main 2020
• You must have scored minimum 75% marks in the qualifying examination.
• There is no age limit criteria to apply for this entrance exam