Curiosity is a gift and it's natural to be curious! Keep it simple.
When my son was little, maybe around the age of your son, he would ask innocent questions. I'd answer matter-of-factly. Sometimes, as he got older, I'd give him more information than he needed (bigger terms, etc.) He'd laugh and say "I don't want to know THAT much Mommy!" as he started to glaze over. Be alert and you can kind of tell when you've said enough. It will usually end with an "okay".
Basically testicles are for making babies. If he wants more, they're for helping him develop into an adult man, like daddy. If he still wants more, they're for helping him to develop whiskers, broad shoulders and seeds for making babies. ... You get the picture. Like I said, he'll let you know when he's satisfied with enough info. It is typically much less information than you might think.
The most important thing is to show your comfort in talking with him about it. And remember they're not asking in terms of sexuality as much as what this part is for. (It's your 5-year-old asking, not your 16-year-old : )
It's also not unusual for a younger sibling to ask things earlier. Keep it plain and simple and I bet that will be enough. If not, keep on answering in a matter-of-fact tone as if he's asking about his nose, or his toes, or his belly button. They're all parts we need to be a complete human being.
And enjoy the curiosity in your little guy! Keeping open lines of communication will build trust for the later years when you get the tougher questions!