I find myself slipping into the same pattern with DS if I'm not mindful of it. He'll start stimming or repeating his favourite phrases over and over or jumping or hooting and people will start looking so I end up spending our whole trip repeatedly saying "stop that .... Stop it ..... DS, stop" until we're both stressed out.
I definitely agree with the PP about parenting the child you have rather than the one you think they should be. When I find myself getting into the "stop it" loop I remind myself that he can't help it, I may as well be cross at him for having blonde hair or brown eyes because he has as much control over those things as he does over his behaviour at times and what I'm really cross at is the people who look, snigger, or comment on it. I'm developing a much thicker skin and I'm now at the point where I will tell people who snigger at him or comment negatively "he's autistic and he can't help it, you on the other hand can".
Being prepared helps, even when we're places that he wants to go. He carries a backpack with a bottle of water in to give it a little bit of weight, the weight helps stop some of the sensory seeking behaviours. I put his 'kit' in there so we have it when he needs it. Ear defenders, some fiddle toys, a snack (always, always the crunchiest apple from the fruit bowl), a chew toy, and his tablet with his headphones. If he starts getting wound up then I can direct him to get something from his bag that will help. We plan in breaks too and I let him do what he wants to do at activities so we don't get into a headbutting contest over me forcing him to join in e.g., at the trampoline park he might want to sit in the café with me rather than go jump even though it was his idea to come to the trampoline park in the first place.
I joined the local ASD support group, they have clubs and activities so we go to a lot of those. It's far less stressful when you're with a group of parents and children in a similar situation to yourself. No one judges at those groups when someone refuses to join in or has a meltdown or randomly belts out a single line from a specific song over and over because we've all been there.
What specific behaviours is he displaying that are causing the problem?