You can claim him if you provided more than half of his support and his gross taxable income was less than $4,300. 

For clarification,  even though my brother filed his 2019 taxes in 2021, I can still claim him as a dependent on my 2020 taxes which I will file next month?

Yes.  Each year is separate.  It probably won't help him get the Stimulus payment by filing 2019 now.  It's too late to use 2019.