K.P.
Good for you in being proactive!
Your son has Autism, so to the best of your ability try to think like he does- literal and sequential. I would be honest with him and avoid euphemisms and metaphors, as they are likely lost on him. I doubt that he sees his social difficulties as a "gift" so I would go the "neuro route"- this is how I explained it to my spectrum clients...
Everyone's brain is built differently. Some people are really good at reading and writing, some are really good at math or sports or art. For you, you are really good at ______________. People also have things that are really difficult for them. What do you think is really hard for you? (Then let him respond and acknowledge his responses). I've noticed that sometimes you have a hard time making friends and that certain _______ (insert sensory descriptor) really bother you. When kids are really good at math and science, but have a hard time making friends and handling change it's called having Autism. Autism means that you think in a unique way and that you always have. Do you have any questions?
Let him guide the rest of the discussion. Making the point that none of us think exactly the same way is "normalizing" for him. Keep in mind that he is likely highly aware of his differences and may be relieved that it has a "name".
Good luck!