Hello again, and thanks for your helpful reply.
The twitching of Ries' back may be due to a neurological condition called feline hyperesthesia syndrome. It's often called the' skin twitching' or 'skin rippling' syndrome. It's likened to a type of seizure. Your regular vet should see him in person and might refer you to a veterinary neurology specialist, if necessary. Here is more information:
Sometimes, the skin twitching can be brought on by stress, so that could be exacerbated re: the addition of the new kitten. Get some Feliway plug in diffusers; these emit cat calming pheromones that have no scent to humans, only cats detect it, and it's great for new introductions.
I don't know how you first introduced them, but this webpage has some good tips and techniques for new cat introductions:
I hope all will be well with Ries!
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