It depends on the factor of autism he has. If he is on the lower end of the scale, it might be best to let him know there is just something special about him. If he is a high functioning autistic, let him know in the calmest, sweetest way possible. Let him know that there is absolutely nothing wrong with him, he was just born with something that may make certain aspects of life a bit more difficult. Make sure he understands the he is not the only one with this condition, and that there are many others out there still living life to the fullest. If he doesn't take it too well, try finding him a support group or a self-love/empowerment program for males/mixed genders.
First let the kid understand that everyone, not just them, goes through something in their life. My dude i learned CIRCLES weren't perfect circles. Some go through depression, cancer, anxiety no one's rlly alone. His/Hers happens to be autimism. But these things shouldn't keep you away from being who u want to be. Then go on and explain the downfalls. Just let them know this struggle isn't bad (death is).
sit him down and explain it to him. How you explain it really depends on how old he is