Weight of the racket depends on the style of the player.
If he's a baseline player, then yes, a heavier racket, particularly with mass in the head will help. It will give more inertia in the swing along with a bigger sweet spot leading to more powerful strikes on the ball.
If he's more of a serve & volley type, then absolutely avoid a heavier racket. You need something lighter with the mass more toward the bridge/grip in order to have more control over the racket for quick reaction and better placement. Usually it's also a stiffer hit that's compensated by a larger head.
Now, with that little "knowing your play style" lesson out of the way... let's move on to thing number two about rackets:
Stiffness of the frame makes a whole hell of a lot of difference. I hinted to it in the second paragraph. The frame itself will also affect the way you hit the ball. Usually there's a correlation between weight and stiffness (lighter composites are generally stiffer), but you never can tell. Here's the thing with rackets - every single one is going to be different. The combination of weight/stiffness/size/string all makes a difference.
When you shop for a tennis racket, if you're a casual player you're not going to care. If you're actually playing tennis for a reason, you use "demo rackets" until you find one you like, then you buy a brand new one of that racket. Not sure if Dick's still does demo rackets or not. Your country club dude might. But for the love of god don't ever just buy a racket based on "oh, it's heavier, so it'll be good". After all, you don't walk into a car dealership and buy a car just because it has more horsepower, do you? Nope... you test drive it first. Can't tell you how many rackets my dad and I have demo'd over the years.