To George, Nick represents the "new wave of the future." It is significant that he is teaching biology for two reasons. First, as a biologist, he becomes a representative of the scientists who experiment with chromosomes, genes, and by extension, our future. George accuses him of trying to readjust our "chromozones." Nick is the perfect foil for George because they represent the opposite extremes in scholarship — George, history, the past, and Nick, biology, the future. Second, it is emphasized that Nick is in good physical condition. This, coupled with Nick's field, concerned as it is with the physical, symbolizes his role in the play and Martha's physical attraction to him. She certainly does not invite him over because of his mental attributes.
Nick is best characterized by his ambitions. While it is true that he is genuinely fond of his wife (he and Honey had known each other since childhood and were expected to marry), he did marry her partly because of her money, which would abet his ambitions. Nick's ambition is attested to by the fact that he even bothers to come to George and Martha's after-the-party party. As Martha points out later, Nick is fully aware that Martha is the daughter of the president of the university, and he certainly did not chase her around the kitchen because of mad passionate desire.
Nick is, therefore, trapped by the events of the evening. He wants to please, but he finds it awkward to stay and watch two middle-aged people verbally cutting each other to pieces. He also wants to please (or satisfy) Martha sexually, but in agreeing to drink with the two of them, he has unwittingly rendered himself sexually impotent.
Nick is blind to the fact that his wife is frightened to have children. Basically, he treats her as a child. He is constantly concerned about the nature of George's language in front of Honey (ironically, he doesn't make any protestations about Martha's equally strong language). And whereas he will openly flirt with Martha and dance sensually with her, he is offended if George makes even the slightest reference to Honey's sexuality.
Nick, while slow in recognizing George and Martha's child as being a product of the imagination, does finally realize their plight, and, as a result, is horrified by the realization. He does not possess the perception to understand why George and Martha have created the child; instead, he is totally perplexed by the revelation. Ultimately, Nick, then is seen as a male conformist who is caught up in a non-conformist atmosphere (George and Martha's house and party) where even his physical attributes fail him and thus, he finds himself in an inferior position with which he cannot cope.