Yikes!

might be a good idea to put a collar on your dog an attach it to his harness via a carabiner clip(or a couple of clips depending on your dogs size) so you can still walk your dog with the harness but have a backup for when he does escape out of it. as even a harnesses that fits a dog like a glove can still be backed out of by a determined or frightened dog.

its what i do anytime i walk Pixie in town cuz she has a history of backing out of her harnesses an i have tried several different types an styles of them. including a few 'escape proof' harnesses. though she hasnt tried backing out of her harness in quite sometime but id rather be safe than sorry in public places; as last thing i want is her backing out of her harness cuz she got scared by a loud BANG(car door being slammed etc) an chasing her through a busy street...

got her demonstrate what i mean. its not pretty but works
https://i.imgur.com/aODi1Jd.jpg
https://i.imgur.com/FiTUSve.jpg

you can also attach poop bags to his leash via a carabiner as well to make it easier for those popup trails in the store LOL