With the inside I think flossing is more important, especially since you can't physically see if there is stuff stuck.
I ended up having one single problematic tooth that kept twisting after years in retainers. My ortho finally gave up and said what would be best would be to run a wire behind my teeth to hold it all in place. So it's a single thin gold wire behind my first four teeth. I need to floss more because the edges of my gums don't get the same stimulation to keep them strong as the rest of my mouth does, but its small and doesn't hurt. It's basically going to be permanent, and I find it very unobtrusive.
I'd imagine he can't fit brackets behind teeth so my guess is for your son it will probably be a wire, though I'm not an ortho and can't say for sure.
TBH starting with Phase I and everything else, your son is just going to get so used to having random metal and other foreign objects in his mouth it won't even matter. I started with a palette expander in 4th grade and was in retainers and other things to move the problem tooth until senior year of college.
At some point the cheeks toughen up, you get used to it and it's just like "Oh hey doc, what are we putting in this time? Wires? Rubber bands? Chains to move teeth around? Bring it on!

" So try not to worry too much, even though its hard. Just help your son have a positive attitude about it and everything will be fine.
