After a lot of research on the subject I've decided to ask my GP to prescribe Leucovorin for William.
Not holding my breath as in the UK it would be an off-label prescription. Normally this is used for cancer patients to help them deal with the side effects, but it's also been used in autistic kids who have abnormal folate metabolism - the DNA methylation test proves William has just that.
I sent him all the lab tests we have had done up to this point as well as some research papers to support my hypothesis on why I think it will help our boy.
I've also just started using a vagus nerve stimulation therapy and just about to buy red light therapy machine so keep your eye out on the updates on how these are going!