You made the right call by turning this into an overnighter. The only other thing you could suggest is a shakedown day-hike, and let him learn what his load will feel like on the trail. Some people only learn by experience.

If he can carry his load, then your biggest concern is probably him sleeping warm. Since you’re very much UL, and it’s only for one night, would you be willing to carry the extra sleeping bag (quilt? I’m not familiar with EE models.) That way, if he gets cold, you can pull it out and he’ll be warm.

He’ll also be more inclined to listen to your advice on the next trip - and, hopefully, there will be one. Just remember that your main purpose on this first trip is to get him hooked on backpacking, not to turn him into a UL hiker. That comes later.