@missmama12 never, ever post on AIBU on disability. Trust me. It's soul-destroying reading comments on posts on the subject when it's not your own issue - when it is, ouch, and you have my sympathy.
People can't help you when their assumption is that autistic people have an off switch, and can behave like anyone else if they choose. The concept that autism is a variable condition (both between individuals, and in any one person in different circumstances and on different days) also seems impossible for the general public to grasp. Just don't waste energy. Post on SN children, instead.
Sadly, the general public's uselessness is something you need to accept, and planning meals out needs to be done really carefully and with an eye on what your son can manage. Mine has sensory processing problems so his resilience needs to be completely factored in, and then we need to go somewhere familiar, when not too crowded, and when sure the menu will suit him. And it can't be too echoey, and he needs to have his chair back against a wall to reduce sound directions. Booths are great for that. You just need to work out what he can manage, so his own anxiety is reduced.
And part of having a child with ASD is accepting that fuckwits will always, always assume your child is spoilt and rude if they melt down. They'll assume it's a tantrum. Gets worse when they reach 7 or 8 and 'too old'. My son has beautiful manners - school always used to comment of how incredibly polite he is - and hates anyone paying him attention, but when acutely stressed he does speak loudly, and if it's too much he'll try to bolt, or just freeze in place standing and not be able to move.
Our biggest revelation was that a tablet allowed him to calm and centre himself. So now when we get somewhere new he has it at once, and then when the food arrives we remove it. He's good with that as it's the rule. But now I get judgemental looks because I'm a lazy parent who gives my child tablets at meals out to shut them up.
When you're the parent of a child with invisible additional needs, you're in the wrong. Just about everywhere. I currently have cancer and it's been a shock, how lovely and kind and supportive and helpful people are with someone clearly bald, and a bit peaky looking. All very sweet, but I don't really need it half as much when negotiating the world on chemo as I do when with a scared and dysregulated child. And then, I've been met with irritation and judgement.
Should say that it's not universal. I've encountered amazing staff members at quite a few places who have absolutely gone out of their way to understand something wasn't right, and he wasn't being spoilt, and to create a situation where he could cope. But the British public, on the whole, are awfully convinced they know all about autism and that autistic people could just stop being autistic if they tried hard enough.
If only people with those attitudes could just try harder to stop being ignorant fuckwits, instead. To educate themselves, so they understand what they're spouting bollocks about and then desist from so doing. But alas, it seems unlikely. 