Good on you for wondering whether his big appetite is something to be concerned about. It sounds like there could be more than one thing going on at the moment with your darling cat. We seem to have two problems, the possible increased appetite and the sneezing.
I would suggest making a trip to your local vet for further discussion about your little man and to have him thoroughly examined, weighed and possibly to run some blood/urine tests.
There are a number of things that can cause cats to have an increased appetite. Firstly, it could just be that he naturally has a great appetite and is learning that if he cries he gets more delicious food. However, there certainly are some common diseases in cats, such as diabetes and hyperthyroidism, that can present as an increased appetite. They will often have other clinical signs as well, such as increased thirst and possibly weight loss.
Sneezing in cats is most commonly caused by “cat flu”, which is a term used to group different viral and bacterial causes of upper respiratory infections. Chatting to your vet in more detail and having him examined and possible tests run will help to decide if this is the case. There are, of course, many other causes of sneezing in cats. If he does have “cat flu” he may benefit from good nursing care, as well as possibly anti-viral medication and even antibiotics if necessary.
I hope that this has been helpful for you and that a visit to your vet helps get your questions answered. Together, I am sure you will get your little munchkin back on track again.
All the very best,