If there is a gay uncle, or yourself, then I would wait until he asks "how come you and so and so" or "uncle Matt and Joey" hang out so much?" etc? Then I'd explain the basics - that Uncle Matt and Joey love each other very much etc.
Graphic descriptions of sexual acts would be completely inappropriate for his age, and for most men out there who weren't gay men - regardless of age. I say this as a gay woman - I will explain to my children why Mummy and Mama are 2 ladies, rather than tlike their friends a lady and a man. And why Mummy and Mama's friends are two men, two women, or both - that different people love one another, and we all find someone who is right for us, and that it doesn't matter to us whether they like someone who is like them, or different to them - we love them just the same. But they DO NOT need to know what goes on behind closed bedroom doors (ours or others) until they are of an age that it is appropriate (and well, no-one wants to know about their parents sex life really, so thats not appropriate at any age!)
I'd teach him about stranger danger and inappropriate touching - because anyone - male, female, gay, straight, - can be a predator. Regardless of whether he is curious or not - better to be safe than sorry.
But, I would wait until he asks - if he ever does. My goddaughter came home to her mother from school one day and said "When people get married, they are a husband and a wife, right?" My best friend (her mother) nodded and said "yes", "And Aunty J and Aunty K are married?" another nod (Civil Union..but close enough), "so they're like wife and wife?" "Yes". She then nodded, and went off to play - she didn't need to know anything further - just that we were together as wife and wife. And she's 8 (was about 7 at the time). So be careful you don't over think the 'question' from him. What is gay? may be easily answered without sexual detail.