My DH and his brother went to Varndean, all be it many moons ago, and they really rate it as a good school. They both came out with good grades and went on to do marketing and advertising at university. You are right that a college will probably not hand hold your DS as much as a school BUT this can be a good thing. If he wants to go on to university a college setting can be good training for independent study without being thrown right in at the deep end. Hopefully someone with more recent knowledge will be along soon to help you out.
Varndean and BHASVIC are both sixth form colleges though. Varndean college isn't linked (apart from sharing a big field) with the school. I went to Varndean college without having gone to Varndean school first.
That said Varndean was pretty good wrt. pastoral care and helping us with ucas (back in the day when I was there).
City College is the one that is an FE college - and not one I'd particularly recommend if he is going a traditional A level route.