Explain this to the assessors as soon as possible before the assessment and see what suggestions they have.
Just a few thoughts you can practise with him beforehand and see what he's most comfortable with.
Can you break it down to try and find out exactly what's triggering him, easier said than done, I know.
Does he generally have a problem with eye-contact? This could be worse on a screen. Is it better if the person on the screen wears sunglasses so he can't see their eyes? if not, just ignore that idea.
Is it the jerky movement of their image, or an out of synch lips with speech that makes him upset?
Some video calls can make peoples' faces look a bit odd, more like their reflection in a spoon or 'horsey' rather than like the more realistic flat faces we're used to seeing on TV.
If it's actually seeing the whole face on the screen that's making him disorientated, can you work it between you and the assessor so that they can see him via their equipment, but he cannot see them, although he'll need to hear them.
If he can't accept just audio from them, can they relay the instructions via headphones to you, then you can tell him what they've said and they can watch and hear his responses, yet he'll be unable to see or hear them and hopefully feel more comfortable.