While genes play a role (say, inheritance type of the body of his mother), for most people, the most important factor when it comes to weight is how many calories you consume and how much exercise regularly will get.
Even though his parents were both heavy, there are things you can do to avoid the same fate: plenty of activity and strength training can both help to boost your metabolism and build muscles and the more muscle you have, the more calories you will burn even when you're not active, making it easier to stay slim regardless of weight issues of your family!