Considering it's a big spectrum with all kinds of intelligence and ability levels and other personal things, can you tell a bit more?
Some common things: In my experience (and those of others I've met) autism often comes with other neurodevelopmental/personality disorders (ADHD spectrum seems to be common). Often one or several senses are oversensitive (hearing, touch, smell, etc.), so if he or you know what it is you could help to "mute" those sensory stimulations (for example: headphones with music (at a reasonable volume!) to dampen other sounds in the case of sound sensitivity, sunglasses in case of light sensitivity, etc.) If he's capable of social interactions, try to help him in that. What's acceptable and what isn't, and why.
I'm sorry, but I don't know anything about that myself! If it's on one location you could try one of those odor spreading devices which change fragrances. (No clue how they're called!)