When you're 7, your developmental "goal" is to conform to the social standards of the society into which you were born. So tell him to look at the people around him, NOT his brothers, because then he'll think that you're playing favorites, but tell him look at people in the store, on the street, or at church, or to look at you as a parent and then ask the question, "Do you see them talking to people like that?" or "Do you see me talking to people like that?" You can describe some nightmarish scenario that would happen if people did talk to one another like that. Describe to him (in a simple way) why that whining is not how our society functions. (Again, DON'T use family members if you're going to give a "what if". That would scare him)
Or, you can let him see the consequences of what he is whinging about. Take the store example for instance: He's whining about how he wants something, so tell him to go get a shopping cart full of everything that he wants. He'll stop whining, and go get everything he wants, and then when he gets to the check-out, let the cashier scan everything, and let her say the total. Ask the 7-year-old how he expects to pay for all of this. He'll ask you to pay for it, and he might go back to whining at you, but you need to tell him to apologize to the cashier for wasting her time, and then have him go back through the store and put everything back where he found it. I know it sounds time-consuming, but it is one of those lessons that sticks forever, because it will seem to him that he just did something that is unacceptable in our society, and he will avoid repeating the behavior.