The sneezing might be due to a nasal polyp or tumor or a foreign object sniffed up into the nasal area.
You didn't say how old he was but a dental issue can also cause sneezing if a tooth abcess drains into the nasal area.
If you have not had bloodwork done you might want to check liver, kidney and heart function.
Howling often goes along with pain, but some cats will cry because of hormone imbalances and really old animals may vocalize at night due to a congnitive problem.
If your vet has not been able to help you then you might consider consulting a veterinarian who specializes in small animal internal medicine and this site may help you find one
http://www.acvim.org/Specialist/Search.aspx
Hope this helps you!
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