this is something i know a lot about because i have been to so many idiot doctors in my life and PT's/chiros.. they always target 'strengthening muscles' but they fail miserably on the real culprit which is alignment and spine health
as you can see from my pic i have a shit ton of muscle on my frame. yet for me i have disc problems in my neck from years of powerlifting/desk job. plus i believe i am genetically disposed to these problems cause i have small limbs + a curved spine already (i was the kid who always got check for skiolosis) .. the idiots i have been to said the same thing "strengthen muscles" and the funny thing is when i went to PT they had me doing shit that old people did, complete cookie cutter crap when i was doing much heavier weight in the gym.. it made no fucking sense but they are so retarded and lazy that they don't know better
for me the ONLY thing that actually works is bikram yoga which obviously is too advanced for a 5 year old lol..my recommendation for your kid is STRETCHING exercises. google yoga poses that strengthen the spine. have him do those 1-2x per day.. it can be something fun you both do together....