My brother has two felonies that are 5+ years old, one of them is a drug charge. What are the chances he can ever revisit the US if he''s deported?

If the felonies are the basis of his removal (and if they are considered aggravated felonies), he will not be able to return to the US.  Someone deported because of an aggravated felony is basically unable to return, ever.   If he can get the felony convictions removed from his records, there may be a chance for him to return, but that path is legally complex, expensive and uncertain.