This is an opportunity to show that there are many paths to Scouting and that you can join at anytime. Also, advancement is just one method to achieve the aims of Scouting. (The "aims" are character development, citizenship training, and personal fitness. Advancement is just one method used to achieve these aims. The other methods are Ideals, Patrols (a.k.a patrol method), Outdoor Programs, Personal Growth, Leadership Development, and Uniform)
At the crossover ceremony, I would recognize the non-AOL scout's accomplishments, respect his decision, and let him know he can join scouting anytime. They are all making a next step in their life, they are just going in different directions.
Maybe say something like
We honor the accomplishments that you have made on journey along the Cub Scout trail and we ask that you too look back and remember those accomplishments and your many friends as well. Now you have reached the end of the Cub Scout trail, where two new trails begin. Some of you will now take the trail (bridge) to Boy Scouts and some will take another trail. These trails will cross again and again. You can crossover to the scouting trail at any time, at any age. See you on the trail.
Those who earned AOL, "crossover" to meet Scouts/Scout Leaders of the troop they will be joining. Now if the Non-AOL scout wishes to join Boy Scouts let him walk a different symbolic trail over to the Boy Scouts. If he does not, he justs remains seated and watches as his friends head down the scouting trail.
Talk to the Scoutmaster and see if he can attend their meetings until he can "officially" join at 11 or graduates 5th grade in 4 or 5 months. Hopefully, this troop is now making plans for summer camp. I think he will lose interest in scouting, if he is "held back" in Cub Scouts. Would you place him in a Web I den?
All new Boy Scouts have to complete the "Scout Badge" which is mostly a repeat of AOL requirements anyway. He will have to work a little harder to complete Scout Badge than his fellow scouts who earned their AOL and hopefully that will make an impression.
Good Scouting