So sorry to hear what your son is going through.
Everyone on this site is different, and has different symptoms, though we mostly have the same diagnosis! But your son’s symptoms do sound quite typical.
It’s so difficult to give advice, and again we’re all different, so what works for one person might not work for another. Obviously he’s already doing medications.
The things that have helped me are really small, for example powdered ginger spice. Research has shown that ginger can be more effective in migraine treatment than medications. I have a ginger tea (literally 1/2 a teaspoon of ginger with hot water) every day, and it really helps.
Another thing a lot of us take is a magnesium supplement. Also taking a walk in nature… obviously at the moment it’s difficult for your son to walk… but in the future, when he can, he’ll want to get some exercise, get his body moving again. There’s obviously been a huge disruption to his balance function, and his body needs at some point to start moving again so it can re-learn how to be upright and stable. It sounds like his eyes are currently carrying the weight of his balance-function, hence they become easily overwhelmed (“visual dependence”/“visual vertigo”).
I think with VM, the brain/central nervous system - for whatever reason - has tensed up, gone into high alert and become hyper-sensitive to everything. It needs somehow to calm down, release the tension and return to normal, which is a long, slow process.
It’s all very unpleasant, but he’ll get through it and be happy again in the future. Stay positive!