According to the Fright Night wiki, he was a vampire servant:
A vampire servant is someone who has been bitten by vampire and made
into a loyal protector. They are not made into full vampires, but they
have great strength and seem to survive nearly everything a vampire
can. They can survive in the sunlight like an ordinary person.
While there is a clear distinction between vampires and the
more-than-human ghouls that serve them it could be said that the
vampires that are recently turned by older, more powerful ones like
Jerry or Regine Dandridge are beholden to them. This master/ disciple
relationship makes the younger vampires the thralls of their master's
will much like the mortal ghouls Billy Cole and Bozworth.
Given the paragraph above, if given the choice I think I'd rather be the servant!
It is worth noting however that the above paragraph doesn't reference anything. The question was also asked over at the IMDB in the FAQ for the film, with the answer stating:
At one point, Peter Vincent notes that Billy can walk about in the
daylight and proclaims that he is human. Near the end of the movie,
however, Billy disintegrates into sand and green goo, definitely not
human. Most viewers conclude that Billy was some kind of ghoul or
possibly a 'half-vampire' who hasn't yet been fully turned. Another
possibility is that he was a golem, made by Dandrige from sand and
other matter in order to serve as his protector.
This seems to suggest that there is no definitive answer.
I like the idea of the vampire servant and how he has only been partly turned as it appears to draw similarities with characters like Renfield and Jonathan Harker in Dracula - being subservient and yet not fully turned.
However - it is never explicitly stated in the film and hasn't been explicitly addressed since. The most common answer seems to be a ghoul/familiar (i.e. half turned servant), with the only certainty about him being that he isn't a vampire as he can withstand daylight.