I'm going to be brutally frank here and say that I don't really have a definitive answer to that. I'm sorry.
Part of it is sheer lack of personal experience; my parents were both varying shades of homophobic, so I didn't have open discussions with them (about sex or anything else) when I was growing up, and I had to work it all out for myself.
To be quite honest at 18 I'd have flat-out rejected any attempt to talk about sex, safe or otherwise, with my parents. (I don't think that's unique to gay teenagers!)
In terms of personal safety — sexual and otherwise — there are some good links here. This site is set up by a charity in partnership with local NHS organisations in Kent, but the links and advice from here are general and will apply to anyone (i.e. if you don't happen to live in Kent). For example, there are links to advice resources with the Terrence Higgins Trust, the LGBT Foundation, Stonewall, Schools Out and the Proud Trust.
You might also want to have a look at FFLAG — a UK charity that supports the families and friends of LGBT people.
At a family level, I think all you can do is keep the lines of communication open, and accept that this may feel like a one-way street at the moment, and that he may not talk to you about stuff that feels a) unique to him and b) highly personal. But over the long term, he will appreciate being able to come back to you, should he need or want to.
What young gay people also need is a community of peers. Not necessarily for relationships, but simply as support and guidance and information. And friends! So perhaps there is a social group he could explore, or a hobby group that is LGBT-specific.