As others have mentioned, my son was smart enough when he was younger to figure out how to get out of the stroller. He would also defeat baby proofing gadgets around our home as well.
My son has Asperger's and when he was younger, he had sensory issues (thankfully, grown out of.) He was also a runner, and a very fast runner. He could make a break from you quickly. Much more quickly than you would expect. When he became too big to carry and could get out of the stroller, we had to use a harness on him.
We would occasionally get dirty looks in public, but we just smiled back and thought "if you only knew...."
Fortunately, the harness helped break him of running. He hated wearing it with a passion. We finally were able to reason with him and say, if you hold our hand and don't run away, we won't use the harness. It worked.
Thank you for your post. We are planning a family trip for Jan 2020 and my third grandchild, my son's little boy has the same issues. It sounds like you are talking about my grandson. He will be 4 when go on our family trip and I'm thinking a harness may be a good idea for him but I'm not sure how my daughter-in-law will feel about it. I'm thinking I will bring up the subject with my son when we are closer to the trip. He can talk it over with his wife, I don't want to start any trouble but I do want my grandson to be safe.
I'm so glad to hear that your son grew out of his issues, I'm praying for the same result for my grandson. We all only want the best for our children and grandchildren.
Pixie Dust to you.