I'm grateful for your concern for your son's spiritual welfare, in addition to all the other concerns I know you must have for him. I wish more parents had this concern; the Bible says, "Train a child in the way he should go, and when he is old he will not turn from it" (Proverbs 22:6).
God understands your son's disabilities and knows they may limit his understanding of some things that you and I take for granted. But listen: The most important thing I can tell you is that God loves your son, and Jesus Christ gave His life for him just as much as He did for you. And God wants your son -- to the best of his ability -- to come to love Him in return.
How is this possible? It's possible first of all because your son already knows what love is -- because he's experienced it from you. Let him know that someone else also loves him -- and that "someone" is God, who made him. Teach him about Jesus, and how He gave His life for us.
In addition, seek other ways to make Christ real to your son. Recently I heard of one autistic child who had learned a number of hymns. Another was able to memorize passages from the Bible. Above all, ask God to help you be an example of Christ's love and goodness to this child He has entrusted to you.