Other possibilities include heart disease, hepatomegaly or lung disease, a lesion in his nose, a nasal foreign body, tooth
or allergies, and
other, more unusual possibilities. The problems could be unrelated. Therefore, he really needs to have appropriate blood work and X-Rays and another exam.
I hope you that your dog is on a grain free, high protein diet for his diabetes and to help him lose weight. Essiac tea plus (available on line) is helpful for diabetes also.