Mama;
To be honest with you I don't think there's a radio easy enough for a 5 yr old to use at WDW , I'm saying that because of so many people using 2 way radios and taking all possible channels available not because the child is not capable of using it.
I personally don't think giving him a radio is a good idea , I have done the parks with both of my kids alone ages 3 and 6 and to be honest I never had a problem getting separated because one was in the stroller and the other one would always be infront of me , but we ALWAYS avoided large crowds. Have you considered renting a double stroller and just have them sit down and strapped when the park is very crowded ? One thing I always told my daughter was if she became separated from us to look for a CM and let them know she was lost and she also had her name and the resort we were staying at in her fanny pack.