My pleasure.  Since allergies are common in the general population,
allergies can coexist in a person with MS.  If your problem is the
fish, avoidance should help determine whether or not your oral
symptoms are related.

My son's allergist says that he should be able to eat tuna
but we are just keeping him fish free to be safe.

It souns like you are with fish as I am with milk. I can't
drink the stuff. It gives me a lump in the throat and then
hours later makes me wheeze. However, I can eat a lot of
dairy products where the milk has undergone some processing.

I would suggest having a look at ciguatera or cigateura poisoning, I
am not real sure of the spelling, but have come across it on several
occasions in the south west pacific tropical areas. It appears to be
related to some kind of worm being eaten by small fish which in turn
get eatem by progressively larger ones, any one eating the end product
have the most incredible symptoms, and every case is different. It
appears to be some kind of "nerve" agent toxin. In some cases it
appears almost immediately whilst others take as much as a fortnight
to be affected.
I did note that natives often catch fish and leave them sitting on the
sand on the beach whilst observing to see if the flies land and remain
on their catch.
Any fish that flies do not land and remain on, get thrown back.
As a general rule the poisoning has long remaining effect and any
second exposure is often fatal. So I don't know that your description
is actually the same as this, but it has a similar ring to it. It is
not apparently detectable by normal means.
You might examine the subject further, but I would be extremely
careful.
Cheers, Merlin

For what it is worth this sounds very similar to a poison that is
quite common in the south-west pacific. Small worm kinds of creatures
appear on the coral reefs, small fish eat them which are in turn eaten
by larger fish. The poison is quite dangerous, as far as I know it is
called cigateura or ciguatera poisoning.
It also involves a kind of neuro toxin which is difficult to detect
normally.
It has strange symptoms which vary case to case, sometimes taking up
to a couple of weeks to appear. It is fatal in some cases.
Anyone affected by the problem should not allow any further exposure
for many years, it can otherwise be extremely dangerous.
Cooking the fish has no effect on the poison. There is supposed to be
a method for testing where a silver coin is placed in a boiler with
the fish whilst it is boiled, if the coin turns black the fish is
poisoned, but I don't know the validity of this method.
The natives have a main method where they leave the fish on the beach
for a period and observe to see if flies land and stay on the fish, if
they land and leave, the fish are thrown away. I don't know if there
is any truth in this method but they seem pretty clever with regard to
these kinds of things.
I have heard of symptoms very similar to those you mentioned.
Cheers, Merlin