Christine,
Lyme disease is usually caused by a tick (those little black bugs that try to bite and burry under the skin). So if he goes outside to go to the bathroom he may have pick up one without you knowing. Espeically if you don't have tick warning etc around there. Usually the ticks are carried by deer, but if your live in a city like mine, deer are all over, so no place in the city is safe for a dog. If not, then another dog who has been out in the wood etc, pick them up, and the tick probably hopped on a plant, and then onto your dog.

They are very pesky and can survie for a week or so without a host. I am so sorry that your dog has this. But it's a really good sign that he doesn't have any of the symptoms. I'll keep you and him in our prays, and hope that he can fight the disease on his own.
You may want to check him for ticks from now on, as you don't want to get bit by one of these either. As a human can have the same symptoms and sometimes even worse. Around here we have tick warnings as well as tv commericals on how to remove them, and to put them in a little baggy and get them tested. The best thing you can do, is try to remove the whole tick with a pair of tweezers. As the lyme disease is in it's intestines so even if the head stays, which it sometimes does, as long as you get the body, you or the dog are generally okay, and won't have to worry about Lyme. But always test the tick, just to be on the safe side.
Every day, make sure your in long sleves and long pants, and start to brush your dog and lift up it's fur, and actually look at the white skin for any black spots that are slightly puffy. Sometimes the spot will swell a little, like a zit. Sometimes there will be a little bit of hair loss for the dog at the bite site, and sometimes all you'll see is a black speck. But you need to get that tick before it starts to break down and go into the dogs skin. As long as it's a pretty fresh bite, you don't have to worry much about Lyme disease, unless a weak immune system or the person or the dog has already been infected once.
Once you get use to knowing what to look for, it will take less then 20 minutes (on a willing dog) to check. But check everywhere, even under arm pits or whatever you call them on a dog. No place is tick, unfriendly.
The long clothes is what is recommended on TV along with rubber or latex gloves, so if one does jump on you, it won't find any skin to dig into. Also do this in an area, where there is title and hard stuff, no carpet, as those darn things are a master at hiding and jumping. It's best to do it outside, but since I am assuming your dog got it, being in the back yard, that might not be the best place to check him. And yes it's hot here, but these ticks are known to bite human flesh just as fast as dog or deer flesh.
Or this maybe a competely one time thing. And I really hope it is for you and your dog. As I have no idea where you live or if there are any deer around. I know you said the city, but I see deer running across the college campus in the fall and to get there, they have to cross a six lane freeway. So cities don't always keep them out.
Hope that helps,
Kit