VT is a pretty fuzzy concept still - almost anyone can do anything and call it VT, so it's good to see your optometrist not leaping in without thought. You really need to do your homework and check credentials on this one.
There is some evidence (but only 1 trial) for VT with convergence insufficiency. There is some practice in using it with accommodation insufficiency and it seems helpful, but this not been properly tested in a trial. Last I looked, there was no research on using VT to help much of anything else. So if your son's issues aren't convergence insufficiency, or at least accommodation insufficiency, there's no evidence for VT.
I can understand why you find the sudden onset unsettling. For what it's worth, kids eyes do change rapidly around this age, and many friends have told me that their kid's prescription needs strengthening every year for a while. So perhaps it's just normal rapid change in visual acuity has exacerbated a problem previously compensated? We found DD's visual issues around 9, and I remember the optometrist shaking his head in amazement explaining that he could tell when testing her that she was seeing double images, and then through sheer force of will making them merge into one after a delay - he'd never seen a kid manage to do that before. (It has been suggested that DD is a tad - - - determined.)
