Hachi's sneezing could be a caused by a couple of things such as an upper respiratory infection, environmental irritant (dust, pollen), or allergies for example. Given that he is otherwise acting normal, I would monitor him for now. Should the sneezing continue over the next couple of days, it increases in frequency, he starts coughing, his appetite decreases, he becomes lethargic, and/or there is a discharge from his nose or eyes, then I would take him into the vet for a health check. Good luck, I hope this helps!