He will likely have this kind of poop until you introduce solid foods into his diet, which will "firm up" the poop.
It's leaking because it's so watery, and the only thing you can do is make sure he's wearing the correct size diaper, try folding the top (front and back) down about 1/2" before you fasten it, pull out the elastic around his legs to make sure the diaper is fully open to work to it's best advantage, and change him as soon as he poops. I found that Pampers with Dry Max work the best at absorption, and to save money signed up for an Amazon Mom account (it's free) @ Amazon.com, and use the "Subscribe & Save" feature which gives me 30% off and free shipping. It really saves me money on something we have to have.