The TOWL-4 usually takes about 60-90 minutes to complete all sections, so 3 sessions could have been within range, if they were each about 30 minutes.
He may find spacing and sizing easier to maintain if he writes on square (graph) paper. Pick a grid size that approximates a legible version of his normal letter size. Or you can try placing a sheet of dark-lined square paper behind his worksheets, to use as a visual guide. Or writing on the backside of one-sized square paper, so the grids are present, but not in the way of his actual writing. (You can make your own; there are many free printables available. Search "free printable square or graph paper". Here's one:
https://mathbits.com/MathBits/StudentResources/GraphPaper/full%20page.pdf)
His fine motor may very well be lagging further behind, if he is not progressing, while his peers are advancing. Although the two tests are not directly comparable, I would agree that it is worth examining the drop in performance further, such as with the re-test you already have planned.
I would speak to the OT regarding interventions. Often they can teach him exercises for hand strength or upper body stabilization, which may help. If the school OT isn't helpful, you may be able to get a referral through your PCP. Just because the neurologist believes it is structural/anatomical, rather than neurological in origin, doesn't mean it can't be improved at all with correct practice. Long-term, of course, it will be more valuable for him to become a fluent typist. (One of my sibs, who also has some level of ligament laxity, still has handwriting that looks like that of a 3rd grader. My parents started that sib on the typewriter (!) at age eight, which, in retrospect, clearly makes more sense than Palmer in the STEM field in which said sib is employed. It took a certain level of relentlessly-polite-yet-stubbornly-assertive advocacy, especially in that pre-ADA era. Also radical acceleration shortly thereafter to university, which made the argument moot.)
