It would be better if you stated his age and his relative age. The suggestions will be more age appropriate.
I agree with the other poster. You can buy a cheap phone like tracfone and use it while you are here. Tracfone comes with ten minutes and you can buy extra minutes.
Walmart and
Best Buy sell these phones and other places do, too. You may get lucky and someone may help you with getting started with the phone. Maybe, you can get one where you live for the US and be ready before you arrive, here.
There are GPS devices that may help you find him if he gets lost. I believe you wear a bracelet on the arm and then you can locate the person somewhat by the GPS tracking. I would find one that is more secure; so, he can't take it off easily. This is a little expensive, but I would certainly do this if I was visiting somewhere foreign. They are used for example for adults who have Alzheimer. You can get started with this device back home; so, your son can get use to it.
I would have current pictures of him that you can give a CM in the worse case scenario. On the back of the pictures state his name, description of his abilities, your US cell phone number and so forth.
Anyway, if he's younger I would highly recommend a stroller with security built in; so, he can't go off. There are some larger strollers available for older children. If he's older, you may want to consider a restraint. A restraint can look nice with a stuff animal concept with it. They strap around the waist and you hold the other end. There called a child harness, here. You can use both the stroller and the harness, too. You can get a longer gait belt (used in physical therapy) and hold the lose end or hold him at his waist. I think the harness is the best idea; I think it would be better. If you have these harnesses back home, I would start using one when you go out in the public; so, your son can get use to it. There may be some resistance to this; so, you want to get him use to it ahead of time. I would visit a zoo or some other public area like a museum with him at home and practice all these techniques back home.
I'm not knowing how many are in your party, but I would try to surround him at all times if he's walking. Have someone in the front and back and on the left and right sides. If you can surround him that will keep him somewhat contained. In some ride queues, you may be only able to fit someone in the front and in the back of him. The ones on the sides should be holding his hand. I would practice at home if he's not use to holding hands. You don't want him to resist holding hands; so, get him use to it back home.
With the parks, I would use fastpass as much as you can. Learn about fastpass, if you are not familiar with it. There are companion restrooms where the whole family can fit. They are somewhat separated from the other public group restrooms, but they are in the same area. You may have to wait your turn longer using a companion restroom, but it's so much more secure. It's unisex; so, everyone can be together. I'm not quite sure if a GAC (guest assistance card) would help you or not. You can read and learn about them and then decide if this may suit your situation. You need to go to guest services at the park to get them. You just need to do this once for your complete stay; you do it at the first park you visit. You will need to be extra careful doing a parade or fireworks; the crowds are unbelievable. I would insist on holding hands, here; unless, he's in a secured stroller.
I think it's critical that you get into this before you come here and use all the aids available back home; so, you will be prepared. Children can be very resistant to new ideas.