Welcome! I am Dr. Altman, a licensed veterinarian and I am happy to answer your questions.
This does not sound like a head injury but possibly something such as a foreign body is under his gum line or in his nasal passage on that side. If you are able to do a thorough look through his mouth that would be great.
Monitor for any nasal discharge, particularly on the side he likes to rub his head and if it progresses to color (green, yellow, red) then it is time to have him seen
Also take a close look at the white of his eye on that side. Often if he took a tumble or hit his head hard the white of the eye is reddened (more than usual for a Boxer) comparable to a black eye in a person
Massaging his head see if there is any spots that are more tender, have any swelling when comparing to the other side, or has increased heat. If these are seen, he becomes lethargic, the sneezing is not improving, or he goes off his food it would be best to have him evaluated
One other thing you can try just in case he did get something up that nostril is taking him into the bathroom with the steam of the shower. This dilates the nasal passages so that when he does sneeze perhaps whatever might be up there is sneezed out
Please let me know if this information makes sense and any additional questions I can assist with today
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Luckily there are no big nerves that run along that area, primarily below the eye and along the masseter muscles so hopefully he did just have hit there and will be back to normal tomorrow. Definitely still monitor that eye, make sure the pupils are equal and reactive to light
You can use a cool wet rag placed in the freezer for an hour over the region of discomfort for ten minute intervals the rest of the day to relieve some of the discomfort if he will allow you to
LOL sounds like a Boxer!