Your son is not going to "get lost." You may lose him. Communicate constantly with other adults in your party about who is going to have eyes or hands on him at all times. Someone should ALWAYS be watching him and everyone should know who that person is at any given time. This is how you keep track of kids. Don't get distracted by your phone, or anything else.
I understand the fear, as my youngest has apraxia. Both my kids are also autistic and neither has ever gotten lost anywhere because it was SUCH A MAJOR FEAR of mine that I made sure it never, ever happened. We were vigilant, perhaps to the detriment of our own enjoyment at times, but that is what you have to do.
I visit
Disneyland several times every month. I cannot tell you how many parents simply stop watching their kids or have their heads buried in their phones and don't notice their kid wander off. This happens a LOT in ride queues. These parents will move forward and the kid hangs back and no one notices.
Similarly, if you are going to do something like leave them sitting in a stroller while you make a purchase, use the bathroom, or anything like that, STRAP THEM IN. Kids will just get up out of a stroller and walk away very often, and their parents take awhile to notice. Our rule was always, if their butt was in the stroller seat, they were strapped in. No exceptions.
Just last week, I saw 4 kids "get lost." In 2 cases, the parent didn't even notice the kid was gone until someone nearby said "is this anyone's kid?" The other 2 were older sisters who approached a CM.
Seriously, just pay attention and you won't lose your kid.