How severely affected is your son?
I volunteer with a summer program for kids with disabilities. There's a few NT siblings, a whole horde of ADHD/LD/quirky kids, and a couple LFA kids, ranging from 5-10 years old (the majority are 6). We explained LFA as just 'having trouble talking' and 'having trouble learning', and explained some behavioral issues as 'he doesn't like change' and 'he doesn't like noise'. Stimming was 'it feels good for him to do that'. I mostly just explained as the kids asked - the questions were things like 'why does he talk like a baby?' and 'why is he yelling?' and 'why is he always bouncing on that ball?'
Explaining higher functioning autism would be trickier, because the signs are more subtle. In fact, I'd be surprised if 5 year olds would even notice anything different about a high functioning autistic kid. I guess you could explain the odd behavior in the same way, and with the social skills issues, maybe ask the kid if he's noticed that he can tell how someone is feeling from their face, and say that X has trouble learning to tell what people are feeling. And maybe ask if he's noticed when people say things like 'it was raining cats and dogs' they don't mean animals are falling from the sky, and say that statements like that are confusing to X because he thinks you're saying that animals are falling from the sky.