My daughter, under the direction of a professional therapist, dealt with a particular fear that was causing her to panic by this method:
1. Discuss the difference between reasonable and unreasonable fears. Reasonable fears are things that could happen, such as something on the stove catching on fire. Unreasonable fears are things like a giant anaconda snake being in your bed (unless you live in the Everglades or something - you get the idea). Your son's fears about getting lost are reasonable - people lose their way frequently. Reasonable fears are things like tornadoes in Oklahoma. Unreasonable fears are things like the teacher telling you to remain in your seat as she leaves the room after school, while the school gets closed for the night and you're left in the school alone overnight. It COULD happen, of course, but it's not something that one typically experiences, as opposed to getting lost, or getting sick, or not seeing your mom or dad for a moment on a busy street or at a county fair.
2. When the fear strikes, evaluate. Is this a reasonable fear or an unreasonable fear? Could I potentially lose my way on the beach and not know where our blanket and picnic set up are, among all the other beach goers? Completely reasonable (happens all the time). If I go to football practice, is there a chance that the coach will suddenly decide to load all the players on a bus and go for ice cream without even looking at the field to see if every player is accounted for, or not notice that I'm standing there? Not so reasonable. Adults in charge check and count the kids they're responsible for.
3. If the fear is unreasonable, have a phrase to tell yourself. "This is not a real fear and I am strong." Practice relaxing breathing (breathe so the abdomen rises and falls, not the chest; this is something you can learn by lying flat on the floor, placing a book on the chest and abdomen. The book on the abdomen should rise and fall, not the chest).
4. If the fear is reasonable, have a plan. On arriving at the beach, look around for a landmark near your site, like a numbered life guard station, or the blue bathroom building, or something like that. If you get confused on the beach, stop, relax, look for your landmark and head that way. At a theme park or public place, have a "meet up" spot in case of separation. "If you can't find mommy, sit under the big clock that you can see from literally everywhere at the fair and mommy will head there." Or have a plan for identifying the park employees and approach those (not just any adult). In the kitchen, learn how to use a fire extinguisher or have a family escape plan and a meeting spot. In a daycare setting, if he doesn't see an adult for a moment, have him sit down and say a poem to himself or count backwards from 100, while breathing properly. Chances are the daycare provider is simply in the bathroom or tending to an infant - most daycare providers will be reliable. Role play these safety plans with him.
5. We were told to respect her reasonable fears, address them calmly, and not to feed the fears or disregard them. When she experienced a panic moment, I was to check for what she was panicking about (with her, it was a medical trauma that actually happened. I was to calmly check her throat, and calmly tell her, "no, there's no bleeding" and then resume normal activities, like go back to washing the dishes or whatever). If your son cries and says he'll be abandoned at daycare, don't tell him "that's ridiculous" or go through a lengthy process to reassure him. Simply say "if you don't see Miss Patty Provider, just do what we practiced. Sit down in a chair. Wait quietly. Say the little poem we learned to yourself, or count. Give Miss Provider a chance to come out of the bathroom or finish changing the baby's diaper or talking to an adult at the door."