What kind of a question is that to be posting at 11.24pm?? You obviously had a bad day!!
I have a child with autistic spectrum disorder, severe communication difficulties and sensory differences.
Interactions usually go like this..... He looks completely normal. Then you would begin to see 'unusual' behaviour such as repeating TV commercials to himself, or giggling to himself etc. Then they might try to talk to him and he would appear deaf. Then they move away. He really is quite high functioning, but I think it is about peoples conceptions when they meet you. When they begin to notice something is different they don't know what to do or how to behave themselves, or they are unaware that they have difficulties and are judgemental about their behaviourt.
So far my son doesn't show alot of inappropriate behaviour. But he can have tantrums in public places. And people like to stare. No-one has ever said something to me yet, and I would give them a mouthful if they did and send them packing with their tail between their legs. How dare they!
Because of sensory difficulties I never go out of the house without his Irlen glasses, DIY ear defenders (to screen out noise) and something he can hold to fiddle with. So I think he does look like 'maybe there's something up with this kid', which sort of gives a warning to people. I think that is useful, otherwise their expectations are too high. On the other hand I have had professionals advice me that I should be starting to wean him off these supports because they make him 'look different'. But I think a blind person looks different with a white cane, but I would never suggest they get rid of it because it makes them look different, and then watch them stagger headlong into traffic. So I would say that if there is something that helps your son cope in social interactions to let him have that support. It might just be something he keeps in his pocket that he can fiddle with.
If it is someone who is going to have regular contact with your child then some information is going to be useful. But I know myself that I have reservations about telling people of his diagnosis. And even if I do tell them what will that mean to them. How can you condense all the information you have into a short note??
I suppose I tend to tell people what might be problems for him if he is going to a friends house for example. But it can all be very hit and miss.
You would have to specifically think about what the issues are with your child and who it is that they are interacting with. Carrying some kind of card could be useful in emergencies. For example I know some autistic people carry a card saying what their diagnosis is and who should be contacted if they are found in an upset/anxious state.
I don't know how old your child is, but I found 'Play Therapy' very helpful. But you have to find a Play Therapist who is willing to work with your child 'because of their diagnosis' and not because they have some kind of 'emotional trauma that needs working on'. I found a Play Therapist who works with groups of ASD children to give them positive outcomes of social interaction and to help them improve their social interaction skills. Since going to these sessions his ability to interact with adults and children has much improved and he actually wants to play with people.

I've just read through my post and realised I haven't answered your question. Do what you can to help your child cope in social situations. But you can't do much about other people. They usually fall into two categories. Those that pick up on their differences and are understanding and accommodating and those that are blind and judgemental and unaccepting of anything other than the norms of social behaviour. You really can do without the latter. Don't waste your time and energy on people who are not worth it. None of us have the time or energy to go on a worldwide crusade. There are good people out there. Try and get your child into groups and social support structures that recognise his difficulties.