Dogs like us will have hearing loss with age, but 7 is too young. Other than an infection or damage from an infection, you might want to think about how you hunt with him. I have seen dogs next to layouts that are allowed to be far enough forward that they are likely in the blast zone of the shotgun muzzle. Even worse is hunting from boats like a canoe or kayak with the dog on the bow or in front. My paddled/poke boats are all configured for the dog to sit behind me. As well I believe to keep a dog out of the blast zone is one of the reasons for sit to flush when upland hunting flushing dogs. http://www.msstate.edu/web/media/detail.php?id=1812
There are of course worse less common things that cause hearing loss, but hunting and age is the most common. My older dog Blaze has hearing loss beyond normal for her age that was sudden and still left over from a tick disease as well as blindness. Which if the hearing loss was sudden, your dog has been exposed to ticks in the last few weeks, and everything else was ruled out, adding that to a blood test might be worth doing to rule it out.
"Onward up many a frightening creek, though your arms may get sore and your sneakers may leak. Oh! The places you'll go!" ~ Dr. Seuss