A quick google of "b vitamin deficiency fine motor impairment" brings up B 12 repeatedly in references. Some B vitamin or other was one of the supplements medically recommended for children with autism that I gave to one of my sons when he was about eight years old and his handwriting issues improved. (I would have to try to dig up old notes, if they still exist, to say exactly which b vitamin.)
https://neuromuscular.wustl.edu/nother/vitamin.htm
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4476750/
https://www.symptoma.com/en/ddx/fine-motor-coordination-prob...
https://www.b12deficiency.info/blog/tag/poor-motor-skills/
https://en.wikipedia.org/wiki/Vitamin_B12_deficiency
(I am not a doctor. This is not medical advice for anyone, much less for the OP. This is just a general observation for anyone interested.)
Edit:
Yep, my old notes still exist and B 12 is one of the three B vitamins listed (I never found DMG but gave the three B vitamins and magnesium to my son and after a few months saw improvement in social issues and handwriting). Excerpt from an old blog post:
"In 1996, I found some information on the Internet about a vitamin therapy that doctors sometimes prescribe for autistic spectrum children. Recently, I found the crumpled, pathetic slip of paper that I originally wrote my notes on. My original note has the following bullet points written willie-nillie, all over the place:
(DMG) Dimethylglycine
125mg tablets or capsules
take 2 - 3 weeks ("then add"),
vitamins B6, B5 then add B12/magnesium
Okay, I take that to mean that all the doses are supposed to be 125 milligrams. They did suggest you start things either one at a time or in pairs (and I take my notes to mean that the pairs are 'B6 with B5' and 'B12 with Magnesium') and observe if there were behavior changes with the addition of each thing. I think 2-3 weeks is how long you should have the child on each supplement before adding the next one."