Agreeing with all the great advice (and making notes of book recommendations for us too). DS5 also gets very worried during "scary" parts of books and movies, but not to the extreme.
You may have already tried these, but I often have to remind him of our guidelines, which seems to help calm him down:
If I know something scary is coming up I forewarn him.
If he gets overwhelmed we can pause or take a time out to discuss.
It's just pretend.
He can sit on my lap or cuddle at any time.
We have also taken complete breaks from reading for a month or so when we can't agree on what should be read. We just let him read to himself during that time.
We've also just instituted a rule that bedtime books read by Mom must be more complex and above DS's reading level. Or "with small print" as he says. I cannot read another A to Z Mystery. Not will not -- Can Not. Magic Treehouse has been banned for a while.
DS likes non-fiction a lot. That might be something good to fill in for the time being.
And I'm also not averse to a little trickery, such as reading a book to myself in front of him and chuckling, or loudly telling DH how so-and-so said this was such a great book. (my opinion matters little already)
Good luck.
