VA would be a better option.
The reason I say this...
He will be able to get his meds for little or no cost (not just MS related), all or partial healthcare through VA. Or he may choose just meds from them and healthcare through a private doctor.
If he gets worse; he would need to be re-evaluated; rating would increase and so would his disability payments.
As you know MS medication is expensive and to know that your out of pocket is very little, if any.
He might be able to fight or have an attorney who knows how to fight for VA benefits to request VA retro disability payments back to when he was first dx.
I know when I separated from active duty I was not given a list or notified that if I get such and such within a certain time frame to come back to them and file a claim.
It is sad that I found out about MS related disability quite by accident...
I'm not sure about SSDI...it would be just a disability check with no benefits, correct? Like if he needed healthcare, meds, physical therapy, etc...would be an additional expense?
I'm sorry to hear that he is not doing well.
Is he taking any medication for MS / symptoms?