I can't imagine a family meeting accomplishing much besides more frustration. He'll get bored, start goofing around, you'll get upset, you'll yell, he'll cry ....
At 5, his memory isn't all that long to begin with. Add ADHD to the mix and he's lucky to remember his name, let alone what he was told 10 minutes ago. Having ADHD is very much like having someone lecture you about Aztec merchants while AC/Difficult Child is blaring on your headphones and your favorite television program is on. It makes concentration very difficult. So, things do have to repeated over and over. For my daughter, I found that if I told her to do one thing at a time, it helped. If I wanted her to pick up her toys, I didn't tell her to pick up the toys, I told her to put away her bunny, then have her panda join the bunny so he wouldn't be lonely and so on until the toys were picked up. Same with getting dressed and getting ready for school. One thing at a time. Any more and she would get lost. Even then, it was hard to keep her directed. She wasn't doing it deliberately, it was just part of her nature. Remembering that helped a lot.