Jay was kicking himself. He really wanted a job he applied for, so he didn't negotiate well for fear of losing the offer. Now, he's upset that he undervalued himself. He's just started the job, and it pains him to think that he's working for so little money. He called to ask if he might reopen the negotiations right away and demand more money.
Unfortunately, Jay is out of luck, at least for the time being. He accepted an offer from the employer, and has to live with it-for now. The time to negotiate is before the deal is agreed upon. After that, it's a breach of etiquette to try to renegotiate right away. Surely, he wouldn't appreciate it if the boss approached him the second week on the job and said she wanted Jay to take a pay cut because she hadn't negotiated as well as she should have.
So, what should Jay do? He could keep looking for another job and do a better job of negotiating next time. If he wants to stay with the company, the best strategy is to start working for a raise right away. He should work hard, document what he is producing for the company, and then renegotiate in a few months when he can present a solid case showing how he is producing much more value than what he is being compensated for. Please see my article on how to get a raise.