Hi knittenkitten,
It is probably the corn syrup or the high fructose corn syrup. That is what is mostly in the store bought drinks. He should get tested. I had hives for ten years and it was soy and it's products, plus corn products too.

No matter how well our MS is doing, or not doing, when we overdo it, we all pay. It's a matter of pacing your day. Rest a bit or just sit and close your eyes for ten minutes. It helps.

If I go out shopping one day, I have to stay home the next and rest up. I don't push myself past worn out to exhaustion. Tomorrow you can always do the rest of the job you need to do. Why feel sick and tired? JMO
I hope you feel better now.

I hope you DH finds what he is allergic to. Is he itchy?