Hmmm, Emmaphiladelphia, maybe for you/your family, the problem is the other way around. Vitamin d and magnesium are supposed to work synergistically. Perhaps the problem is low magnesium (which most people have to varying degrees) causing the low vit d level. Of course, I don't know for sure but I would consider that. The level of magnesium shown in just the regular blood tests is not a good indication of true levels in your cells. You would be better off with a red cell magnesium test (see Dr Carolyn Dean's work).
https://www.grassrootshealth.net/blog/vitamin-d-status-improved-magnesium-supplementation/

Thank you for the info Julie.

We live in a climate where there is warm weather and plenty of sunshine. As home schoolers, we were outside all the time. I think there was something interfering with his system to cause the drop in levels. I personally witnessed this rapid turnaround in his health/mental state after just a few days on this vitamin D3 dosage. Perhaps it was just a miracle. I had him continue the vitamin regiment for the three months and after that he refused to take any more. His symptoms never returned. As a young adult, he is still living with us while he works and saves up to eventually move out. He refused to take vitamin D3/zinc/C to protect from Covid. He was in a job that was never shut down, so he had ongoing exposure to the public. I did take the regiment and my spouse took flaxseed oil. The only time all of us got sick was when "omicron" was making the rounds. This son got sick first and we got it a few days later. He got over it in a few days, and my spouse and I had about 3 bad days and completely recovered
after 3 more days - enough to give us immunity. Make of this what you will.

Emmaphiladelphia, keep in mind that there are many doctors, including Dr Mercola, who believe the levels that are quoted as adequate (20ng/ml or 50nmol/l) are not nearly high enough. What is considered 'high' (40-60ng/ml or 100-150nmol/l) is what many others consider 'adequate'. I was sitting at about 100nmol/l and I had a doctor who is well versed on Vit d tell me I needed to get to at least 150nmol/l). I have read also, that if a person's Vit d level is low, more magnesium supplementation is needed. If the Vit d level is high, less magnesium supplementation is required. Grassrootshealth.net has some good information. I think the best way to get good levels is spending time in the sun with as little clothing as you can get away with (not to the point of being burnt of course). I believe Mercola says the best time is around the midday sun which is not what the 'experts' tell us but we know how the experts often get things wrong. The sun provides more than just Vit d.

Today I’m bandaging my son’s small injury while he curses in a loop & watches Barney.
There are so many tangents of emotional & physical trauma connected to living with someone with “severe autism”. The worst for me is worrying about finding competent & caring staff to be my son’s “family” for after I die.