He might be a foster dog, a visiting dog, or a newly acquired dog, but how or why a dog is now in your house isn't important. If he's hiding and displaying fear and avoidance tactics, it's going to be a problem. Especially if your home is the fearful dog's new home. You can overcome this shyness, but your new dog can't be rushed into things. Prepare yourself to spend at least an hour or two, if not more, on this first meeting.
Present An Unthreatening Appearance
People, just by being human, loom over dogs, towering from heights that make a human seem very scarey to a frightened dog. Lessening this effect, however, can be as simple as crouching down to eye level of the dog you are trying to attract. You may also wish to arm yourself with an array of tempting treats, preferrably ones with a strong scent.
Now you wait for the dog to get curious enough to come to you. Don't make any sudden moves, or loud noises, just lie or sit patiently. You can call his name (if he has one) and try to entice him over, but any quick moves or attempts to grab him will only cause him more fear. All you can do is wait him out.
It's Still Not Working
If you've exhausted your patience and he's still not showing an interest in you, it may be time to recruit help. A friendly, outgoing dog might be your best option. Often when all else fails, another dog to meet will be just the thing to bring a shy dog out of his shell. Just be sure the other dog isn't dog aggressive.
When He Checks You Out
Don't scare him off with lavish praise (that will come later), but let him take his time and look you over all he wants. Encourage him using soft words and hold out your hand on the floor for him to smell. Any sudden moves on your part may scare him back into hiding.

