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Where on his body is the hair loss?
Can you think of anything that has changed recently? New food? New Treats? New household cleaning products? New dog bed?
Is he itching or licking that spot?
Is the skin red or inflammed?
What size is the spot?
I would be suspicious of demodex mange. This is more common in younger dogs. It usually causes one spot of hair loss. The skin can appear normal, be slightly red, or have some darker spots on the area. It is diagnosed with a skin scraping which is examined under the microscope. If positive, he would be given and oral or topical treatment to kill it. This type of mange is not contagious to you or other pets.
I would think if it were allergies, there would be itching and redness.
There is a condition called seasonal flank alopecia. This is where the dogs loose hair, on both sides of their flanks, in the winter time. But it is on both sides of their flanks and not just one.
www.VeterinaryPartner.com/Content.plx?P=A&A=630 here is a good article on demodex mange
I hope this helps.