I’m sorry to hear that Greyson is sneezing, and seems to be less active compared to usual.
I suspect that he has picked up a contagious upper respiratory infection. An upper respiratory infection in cats is just like a cold in you and I, and these are commonly caused by a virus, the most common one being Herpes virus. These are easily caught by breathing in virus particles in the air from a sneeze or nasal or eye discharge. Some cats get sicker than others and young, healthy adult cats seem to tolerate them and fight them off better than very young kittens or older cats. Symptoms can range from mild sneezing and lethargy, to heavy nasal and eye discharge, high fever, swollen, red eyelids, open mouth breathing, and pneumonia.
Lethargy is pretty normal with a respiratory infection.
You can help him feel better by adding warm water to his food to make it smell more (they don't eat if they cannot smell) as well as making it easier to swallow with a sore throat.
Also the more fluids he gets the better. Offer tuna juice, low salt chicken broth, run the tap if he likes to drink out of the sink.
Take him into the bathroom with you if you run a hot bath or shower as the steam will soothe his sore throat and airways.
If his nose become very congested you can use sterile saline to loosen the thick mucous and remove it. He won't like it but it will help him breathe and be able to smell his food better. You can also use sterile saline to remove eye mucous if it accumulates.
You can give him an amino acid supplement called L-lysine at a dose of 500mg orally twice daily. If this infection is due to Herpes this amino acid interferes with virus replication and will shorten the infection's duration and severity. Good supplements to try are made by the Viralys brand which comes in a powder to add to the food or a tasty gel.
Some lethargy is understandable, let him rest as he needs rest to get better. If your cats normally go outdoors keep him (them) inside until they are back to their normal playful selves.
If you want to try Benadryl (diphenhydramine) to dry up his nose a normal dose is 1mg per pound every 8 to 12 hours. So a cat 8-15 pounds could take one half of a 25mg tablet every 12 hours. Benadryl is very bitter and some cats will drool excessively or may even vomit because of that. That doesn't mean it is making him ill, he just hates the taste. If that's the case with him try a different antihistamine, chlorpheniramine at 4mg per cat orally every 12 to 24 hours.
If he runs a high fever (more than 104F), has a green or yellow nasal or eye discharge, stops eating even with coaxing and clearing his nose and eyes, or starts coughing or having difficulty breathing then he needs a veterinary exam. Not an emergency as long as he is eating but soon if he isn't eating or has a green or yellow nasal or eye discharge.
Sometimes these upper airway infections turn into pneumonia so that's what we need to guard against. In most cases antibiotics aren't needed and can contribute to a decrease in appetite so I don't tend to prescribe them unless I feel there is evidence of a secondary bacterial component. These include a green or yellow eye or nasal discharge, evidence of pneumonia upon listening to their lungs or an infection that lingers beyond the normal 7 to 10 days.
I highly recommend testing him for feline leukemia and feline immunodeficiency viruses if his respiratory infection lingers. These immunosuppressive viruses will make a simple infection much worse as they stop the immune system from fighting infections the way it was designed to do.
Other reasons for sneezing and nasal congestion are an infected tooth root, a fungal infection, a foreign body inhaled into the nose or a nasal polyp. If he isn't much better in 7 to 10 days he needs a veterinary visit.
Let me know if you have any further questions.