I'd second (fourth?) this suggestion, with a couple of extra provisos.
First, talk to him about what to do if he gets lost on his own. Of course, you'll watch him like a hawk, but I've never been to a SAR callout where the adults have told us, "Yeah we were really slack, and he was gone for twenty minutes before we realised" Stuff CAN happen, he gets distracted, and next thing he's lost. He could be eight steps from the trail when this happens, and not know it. YOU get down on your knees and see how much less you can see from down there.
Second, tell him all you like, but it means NOTHING if you don't PRACTICE it. Both my kids (6&8)started doing o/nighters when they were 2, and we've played the lost game ever since then. This involves letting them get about ten steps ahead of you, and then you yelling out "LOST", followed by them calling "LOST" You can talk them thru what they should do next. (This should be after having explained what to do) You may be very surprised at how they have interpreted your information. The natural progression of this game is once they're okay with the process, you call out "LOST" but don't help them thru. "LOST", then step behind a tree, so they see you go. We're up to walking 50 yds off trail to explore then calling. They always know I'm close, but they can't always see me. This has to be in their comfort zone, but you will see their comfort zone expand, very quickly.
The purpose of them calling "LOST" at the start, is to make them stop and think, instead of continuing to move.
They ask for this game on almost every hike we do, and love the challenge and praise they get from it.
Enjoy your trip.