Maybe it's too much TV? I agree with other posters that if he starts asking for things on commercials, it's time to turn the TV off for a little break. I assume he isn't watching TV all day, but maybe look at how much he is watching and cut back a little. It also might be that he is trying to get your attention while watching TV, which shows that he wants to interact with you, not passively sit in front of the TV. If you do want him to watch something, put in a movie instead so he won’t be exposed to the obvious advertisements for toys.
Maybe you could also start talking to him about concepts like generosity and giving so that he can begin to see both sides of giving and receiving. Emphasize how you give gifts for birthdays or Christmas and talk about how nice it is to give to others. If you give him a catalog to look at, suggest he pick out a gift for a friend or his brother or something instead of only looking at things he wants. I know at a young age, kids are essentially self-centered, but it doesn’t hurt to start planting the seeds.