1)tax. Most companies have tax equalization programs--this is approved by the U.S. government. The company will pay any tax difference out of your pocket to you so that you are not negatively impacted by a higher tax rate. Effectively you are still responsible only for the U.S. tax rate as if you were still living in the State. But tax equalization is a complicated calculation--his company needs to hire tax accountant on his behalf.
2) Many companies give foreign allowances to expats living in foreign countries. I think allowance can not be more than 25% of his pay in most cases (unless it is places like Iraq). He can ask his company about this.
3) For PR status (that I am), I need to return to the U.S. within every 180 days. Another option is to apply for Travel Document which usually allows the holder to be outside the U.S. for 2 years without returning to the U.S. But the travel document only allows you to keep your GC. The days you have accumulated in the U.S. towards the requirements for naturalization into a citizen will be lost. The third option is to apply for N-470. The requirement on N-470 is that you will need to be in the U.S. physically for a whole year between the time you got your GC to the time of your application without leaving the U.S. for a single day. I did not qualify for N-470 because I frequently travel outside the U.S. I applied for the travel document. However I am still returning to the U.S. quarterly.
4) Some companies have "relocation buyout" programs to purchase your house or to pay for all the cost associated with selling your house. But this is less commom.