If your dog's starting to lose his hair, you're right to be concerned. A range of factors could be causing his hair loss, including stress or changes in his environment. Before looking into environmental reasons, a veterinarian should examine your dog to rule out any underlying issues.
Stress and Anxiety
Dogs are sensitive animals; excessive stress or anxiety in their lives can cause hair loss. Sometimes hair loss is a bodily physical manifestation of stress, but other times anxious dogs will partake in compulsive licking that leads to hair loss in areas they lick too much. Dogs may get stressed or anxious due to a number of reasons, including confinement, bullying from other pets in the house, separation anxiety and past trauma or abuse. If you can identify what's triggering the stress, eliminating it can be as simple as removing the trigger. However, there's a possibility you may have to see a dog psychologist or behaviorist if things don't improve.
Moving to a New Habitat
A change of habitat can be stressful for a dog, in cases so stressful that it causes causing hair loss. Of course, if the dog is a new addition to your home or you've just moved to a new house, there's not much you can do about this change; it may take a little while for your pup to adjust. The best thing you can do is keep things at home as calm as possible and keep a firm routine so your dog will more quickly get used to how things are in his new home.
If your dog doesn't seem stressed out by new living quarters but he's still losing his hair, some sort of irritant in the dog's environment could be causing hair loss. He could have picked up a fungal or bacterial infection from somewhere inside or outside the home, or he may be allergic to something in his new environment. Mold in a home can be toxic to your pets before they become toxic, or known, to you. Your pooch may be allergic to any of several things, including chemicals, such as those in paints or cleaning products, and pollen and grasses.
Even if you think you've figured out the cause of your dog's hair loss, it's imperative that you take the dog see your veterinarian. All sorts of medical issues could cause your dog to lose his fur, including Cushing's disease, congenital hypotrichosis and certain forms of cancer. So you should get a vet to check your pet out just to be sure he's not sick.