Wet or dry? And why does he eat so fast?

We just brought home a 5 month old kitten. I call him Romney-Cat. We give him half-dry half-canned. He eats uber fast like he's never eaten before in his LIFE. Its strange, and I know that he's on a low quality diet, Fancy Feast (dry) & Friskies (wet) - I want to switch him over to something higher quality and I dont care about the price - just higher quality - but what? Should it be wet only? Or wet mixed with dry? 

Knowing my grandparents I dont want him to gain a huge amount of weight there super fast. So, I'm going over there a lot to keep an eye on what he eats and what he does over at their house (dad's allergic so he cant stay at my place.) Any suggestions on how to give him a healthier diet so he doesnt gain too much weight? I've seen some really overweight cats *_* 

Do not free feed dry food, that's usually how cats get overweight. Feed 80 percent canned 20 percent dry. Stick to good quality canned and dry (pet supply store) not grocery store. Try grain free as much as possible although some good quality foods do contain brown rice, peas or sweet potatoes. Look for real meat or fish as first three ingredients not words like meat meal or by products. Look for chicken, duck, salmon (the real animal) listed as first 3 ingredients. Most adult cats eat 6 oz wet or 4 oz wet and 2 oz dry. Kittens eat more small meals throughout the day. Do not feed dry only as it leads to many health issues as they age.

I never planned on feeding him only dry food for his whole life, but I thought if we mixed in the wet and dry together that'd be a good idea because he'd need something crunchy for his teeth, right?

A lot of people feed all wet without issue. If you're concerned about dental hygiene and dental health as a whole, taking up tooth brushing is a better way to ensure the health of his teeth:

http://www.thecatsite.com/a/does-dry-food-actually-clean-your-cats-teeth

If you do want to feed dry, as some people will include it for variety or for budgeting better, a food that is low carb is best - you can use something like this to calculate the carb content of any food using the percentages on the label. I use Friskies in my mix of wet food, but the other "better" wet foods I use tend to be fish, grain, and byproduct free (I don't mind byproducts, some do).

On his papers they say that he's three pounds and nine ounces and @ five months old. Is that normal? I havent had a cat in ten years or so, so I have no idea how this should work O.O 

Also, I dont mind feeding him just strictly wet food.