Speaking as an animal behaviorist and holistic care practitioner with years of experience rescuing and re-homing dogs, here's my two cents: Any newly adopted dog will go through a period of adjustment, and since you probably know little about his former life, it's hard to diagnose how quickly this behavior will change.
You can try giving him Bach's Rescue Remedy (a couple of drops directly on his tongue, or put into his water dish). This is not an immediate cure; it might take a couple of days or even a week for you to notice a difference. There are also more specific remedies (search OnlyNaturalPet.com) but Bach's is the one I keep on hand for general use.
Also, try not giving him his food in one dish. Instead, buy several small "Kong" toys, stuff the food inside (you can seal off the ends with peanut butter or cream cheese), and hide them around the garage for him to find. This will keep him busier and distract him from the fact that he's alone.
Try leaving a radio tuned to a talk-radio program, or classical at a very low volume (although I once had poodles who couldnt stand certain classical composers -- very sophisticated tastes, those French dogs)
It does not sound like his behavior warrants any more extreme measures (i.e. prescription drugs). Give another month or two to adjust, give him as much calming attention as possible when you are with him, and evaluate from there.
Good luck and thank you for adopting a shelter dog!