sneezing is a symptom of Upper Respiratory Disease sometimes called 'cat flu'. symptoms include: sneezing, discharge from eyes and nose, loss of appetite...sometimes mouth and eye ulcers, fever, excessive salivation.
this is caused by a virus, an otherwise healthy cat can get over this in a few days however a cat with a weak immune system can develop into a URI/ upper respiratory infection and than it would require antibiotics....or it will progress into a serious condition.
but never use an antibiotic without CONFIRMATION of an infection it can lead to antibiotic resistance...so always be cautious.
I can send you some names of supplements you can use to help support his immune system.
there is a (human) herb to help with the congestion Do you have a good Health/Vitamin shop nearby?