Andre Higgs

My father is more than 70 years old and has now been staying in the UK for more than two years. He came to the UK with his BNO visa. He does not have a job or intend to find one. He has received certain amount of rental income which is derived from properties in the UK. His rental income is slightly over the personal tax allowance amount (GBP12,500). Does he need to apply a NI number? Does he need to file self assessment forms in tax years? Thanks in advance.