Hi. My daughter has pda - she is 27 now.
Firstly, I would say that anyone who doesn't have a pda child will give you all sorts of seemingly sensible Ideas which won't work.
PDA "tantrums" are better understood as panic attacks. Your goal should be to keep him regulated as much as possible and stop him from getting into panic mode. Once he has entered the panic mode state, you have lost the chance to get what you wanted without a huge amount of trouble.
So, retacing your footsteps, you need to intervene earlier. Pda kids hate direct demands so you can't eg say "right we are leaving". You can offer choices eg "would you like to run to the car or push the buggy?"
You can use sensory stuff to keep him regulated - does he prefer to crunch, chew, suck, lick for example, then have appropriate snacks he can have during the outing or when you are leaving.
Try not to argue with him or chase him. Stand your ground. Repeat the options, offer the snacks. You need to stay calm and regulated too. You might need a snack yourself! Main thing is to not escalate matters. You may have to wait 5 mins and try again.
Try to go to less exciting places at first - a walk in parkland rather than into a swing park, so he has less to distract him.
But also you need to keep his home life similarly non over demanding, because the more he knows you understand and are trying to help him, the more he will be willing to listen to you.
Do you use pda strategies at home? Has he had any sensory assessments to see what helps him to be and stay calm?
Good luck.