I work with autistic kids all across the spectrum. The higher functioning kids and young adults that I work with have a problem with abstract concepts such as algebra, invisible germs, and certain social values. God, salvation, and everything that goes along with it is the most abstract of all. Your brother is not alone by a long shot with this struggle.
One metaphor that might be help when discussing abstract concepts is “air” strangely enough. We can’t see it, but it is there. Cant see it? You must not need it. Hold your breath then! Simple things like that can at least open their mind to the abstract.
You can show him conversations on the internet where people who have experienced miracles by God have happened. If you keep this effort up for a while, he might start taking it on faith. If his mind opens, teach him how to pray, and what to ask for.
Outside of this of course is prayer on your part. Remember as well in the CCC it states that God gives special dispensation for those with mental and neurological conditions. Autistic people often have companion conditions, such as ADD and so on.
God knows of your brothers issues. He also is aware of your efforts to help him and can provide you with invaluable support. Bless you for your efforts, and strength.