I never had a male Cavalier but the girls did sometimes get things like black heads on their tummies, nothing to worry about. Being where they are on your dog I wouldn't recommend squeezing them I don't think he'd like it
I used to have to male Lhasa Apsos and one of them developed these sort of markings on his testicles, as he got older. It's just skin pigmentation and nothing to worry about. They were not blackheads, just dark freckles.