Parents of autistic children... How do you deal with people who don't understand?

Hi, we've just found out a couple of month ago that our 3 year old is autistic, i am sick of people who don't or won't understand and judge my son as a boy. For example, he throws himself about on the floor as this is what he needs to do for his sensory imbalance and he also flicks switches on off on off, of course i encourage him not to do the switch thing but my mum keeps saying he's and he needs a smack, i'm sick of hearing it now. Also when we're out he cries like a baby would do for simple things like when strangers say 'hello', he screams and people outside will say 'you're '. How did you deal with this? Thankyou x