Will he working for the last 8 years without authorization be an issue?

Generally, an adult U.S. citizen may petition in the Immediate Relative visa category for a parent who entered the U.S. lawfully and with inspection. This is true regardless of whether the parent may have overstayed his initial visa and regardless of whether he may have worked without authorization. Especially if you are not thoroughly familiar with the application process, it would be wise to engage an immigration law firm to provide representation. Some immigration law firms, including mine, offer legal services on a "flat fee" basis so that a client will know the total expense from the very beginning, and a few immigration law firms, including mine, offer an initial consultation free of charge.