How can i curb my cat's appetite?

Sorry I have to be very brief with this post. My cat, Levi, is 8 years old and weighs approximately 18 lbs. He's never been able to just graze like other cats do... So I've had to measure out his food but he eats it all in one sitting and is still hungry. When his feeding time comes, he acts as though he hasn't eaten in DAYS. Sometimes he eats so fast that he throws up afterward. And then if course he wants more food.
any suggestions? He is at the vet for a different, more serious issue right now, so I will consult the vet on this matter when I pick him up. Just wanted some ideas beforehand. Thank you

Since he is already at the vet, that will give you the opportunity to get professional advice on Levi's diet, especially since he currently is being treated for a serious medical issue.

Does he eat dry or canned food?  If it is dry, and you continue to feed him dry, I've read that some people with cat's that don't graze will put the food in a treat dispenser to try to slow down the eating.

https://www.google.ca/search?q=cat+...a=X&ei=P-BuVb3mAoWggwSW5YGQCg&ved=0CAcQ_AUoAg

Generally, cats will lose weight more easily on a high meat/low carb wet or raw diet. Cats lack the digestive enzymes necessary to access the protein from vegetable sources, so when fed a dry diet they're more likely to overeat. Also, with wet food of any sort you're able to feed a bigger portion than you can with dry. Yes, a lot of it is water, but that water is needed.

To slow his eating there are all kinds of slow feeder bowls on the market designed specifically for this kind of problem, or you can create your own basic version by placing an upside down bowl (or eggcup if the main bowl is small) in the centre of his food bowl. Be sure not to overfill the bowl too. The more food is in the bowl the easier it is to bolt it.