I would probably try and figure out how much information they are actually asking for. I think sometimes they aren't really asking what we think they are so we can give too much information in our answer so start out really simple and let him ask for clarification - I like Arimeh's 'special hug' description. Kids seem to be pretty happy to accept a lot of stuff as just being the way it is without needing to understand every last detail. I think they are probably ready to hear the details when they are old enough to ask for more information but I usually ask my kids what they are asking before I answer questions. A good way to figure out what they are actually asking is to ask them what they think the answer is and go from there.

Hope that helps.