Can you tell me about weight loss in elderly dogs?

My dog is 13-15 years old. He is a chihuahua mix we found on the street and took in and nursed back to health over 10 years ago. I've noticed lately that the bones on his face--especially the side of his head next to his eyes (his temples?)--are quite pronounced and he seems to be losing muscle mass generally. What next? He has had a serious heart murmur (grade 3 out of 4) for several years and continues to be asymptomatic. He has been peeing more in the house over the last year or two, but that seems more linked to the introduction of a small, loud child into his environment. (He was likely abused and was very difficult to housetrain, but did get there eventually). His eyes are also getting a bit cloudy, but overall he is pretty spunky for a fellow his age. Still likes to go out for walks, has a good appetite and dances around the table at mealtime. His last checkup six months ago didn't turn up anything, except that his teeth are quite bad but we can't do any anesthesia due to his heart murmur. He eats a high-quality dry dog food for seniors.

I've read about myositis online, but that doesn't seem to fit his symptoms. Is this just normal aging? Or might it be something else? Other online searches show muscle wasting in older dogs tends to be in the legs, but for our dude, it's most pronounced in the face with a bit along the spine and ribs. I will call the vet Monday for the next available check, but wondering whether anyone has any experience in the meantime. I know his days with us are probably numbered but would like to know if this sounds normal-ish or more critical. Thanks in advance.