They usually blow their undercoat in the spring and the fall. But it should not go on for a month.
Omega 3 fatty acids can help with skin inflammation and shedding. Fish oil is a good source of omega-3s. One teaspoon, per 15 pounds of weight, twice a day. You can buy it at the drug store or the health food store. Put it on his food. You can also buy ready made preparations such as "3V caps/liquid" or "derm caps/liquid."
He may have developed allergies.
You can try human benadryl at a dose of 1 mg per pound, every 12 hours, for allergies
Hypothyroidism is common as they get older. This means the thyroid gland is not producing enough thyroid hormone. Dogs will low thyroid levels have chronic skin issues. They shed more as well Your vet can take a blood sample and check his thyroid levels. If his levels are low, then a daily thyroid replacement supplement can be given.
To be thorough, he should also have a fungal culture to rule out ringworm which is a fungal infection. This is done by having a vet pluck hairs, from an affected area, and put those hairs on a special fungal growth gel. If he has ringworm, or a fungal infection, he will need topical and oral anti-fungal medications.
Any kind of allergies, or fleas, will cause him to shed more.
Food allergies and environmental allergies would be at the top of the list.
Beef, chicken, lamb, corn, wheat, and soy are the most common food allergens. Food allergies develop over months to years.
To determine if he has food allergies, he would need to be put on a completely hypo-allergenic diet for 6 weeks. The name of the hypo-allergenic diet is "hills z/d." It is a prescription diet and you have to get it from a veterinarian. There are no over the counter completely hypo-allergenic diets. It is only about $10-$15 more a month so it is not that bad if it fixes the skin issue. If his skin improves, then you know it is food allergies.
I feed my own dogs "proplan sensitive skin" which is low allergen but not completely hypo-allergenic and will not work for the food allergy test.
He could also have environmental allergies. This is called "atopy" or "atopic dermatitis." The dog's skin comes in contact with the allergens and then they are absorbed into the skin causing an allergic reaction. Your vet can do a blood allergy test to see what he is allergic to. Then he could receive allergy shots to desensitize him to the allergens. The allergy shots are 50-75% effective. We also treat atopy symptomatically with antihistamines, shampoos, topical creams and sprays, cortisone, and immune suppressing medications.
I hope this helps.