Is it normal for my son to talk in his sleep when he's having nightmares?

Yes. Yes, this is normal and common and not a bad thing at all.
Yes. Children make all sorts of noises when they sleep, even coherent sentences (actually adults do this too!). If your child wakes up screaming they may be having a night terror, which is a slightly different type of nightmare. With night terrors a child may scream, breathe faster and have dilated pupils. They will not remember these in the morning, whereas they will remember their nightmares.