That's tough for you all.

When a young family member was going through some severe health issues I remember thinking I could cope with that, I could support her, it was other family members reactions which were actually more difficult to deal with. No easy answers sadly.

He will go through it in his own way and you may have to keep your own counsel for a while as he deals with this blow. You could maybe offer to research MS with him if he's open, if you then steer him towards reliable sources of information which will give you solid facts.

In the meantime if you can talk this through yourself with someone outside the family you might benefit from an unbiased listening ear.