Did you take time to introduce Seymour to the new kitten?

Has the kitten been vet-checked? If not, Seymour might have caught something.

New pets should be kept separate from the existing pet, in their own room with food/litter box/ etc. Switch favourite sleeping blankets or even cat beds so they get used to each other's scent BEFORE they meet in person.

I really hope Seymour gets better soon. Please write a bit more about how you introduced them to each other, k? It'll help us help!