Which state tax applies if your live in NY and work in California on a job that will be about 3 or 4 month long. Company is located in ILL. Will he pay NY taxes in the state he lives in?
You pay taxes in the state where the work is performed.
If you live in NY, and you perform your services for your company in NY, and you do not perform any work physically in the state of CA, you would not pay CA state income taxes.
You may have to file taxes in NY, but you will get a credit in the amount you paid in California, so it'll zero out.
Therefore if you are working physically in California, then you will pay California income tax.