Bb, same here, ds1 is 11 and has AS. He's already on edge, as he's going through secondary transition and was devastated to leave his primary school, let alone having to deal with going to a different secondary to all his friends and travelling an hour by LEA transport to get there.
As I mentioned upthread, I gave him one of my puppy books (this one) to read, so he could understand why we needed to make some changes, rather than things seemingly just happening for no reason and disrupting his routine etc. We also spent lots of time in the preceeding weeks talking about what it would be like to have a nippy, toy chewing pup in the house and how we could make him feel more comfortable.
He's taken it better than expected. He is tic-ing and his eczema has broken out on his hands from constant washing due to hygiene worries (he has OCD tendencies) but some or all of that could be due to the school transition stress and he does enjoy having Pip here. It's been nice to see him forget his worries for a while and play with or Pip or laugh at his puppy antics.
I think eventually things will settle down and Pip being here will just be the new normal for him, as always, it's the transition that's the tricky part.
Not sure what to advise re reassuring him about the cats though, sorry. Perhaps something about how cat's are very capable of telling dogs off and usually just need to tell new puppies off once and then everyone lives together more happily because the pup knows not to chase them?
Lurcherboy was swiped by my PILs cat today. They've met tonnes of times before and he's always ignored him, but today Lurcherboy was over-excited and stuck his nose up his bum as if he was another dog - cat wasn't amused and you should have seen the look of shock on Lurcherboy's face ... before he ran away with his tail between his legs! 