Federal Taxes: Temporary Living Expenses

My son is from Illinois where he has a rental house. He is an engineer and gets moved with new contracts. He is currently in Colorado temporarily before moving on to West Virginia. The company is paying his living expenses tax free in Colorado since he has a residence here. I believe this can only happen for one year or less to be considered temporary. Does only future money become taxable after that or is everything he earned for the previous "temporary" year back-taxable? Also is there any way he can get rid of his residence here in Illinois and maintain a tax free status? Could he claim my residence as his permanent housing? If he was audited on all this tax free money, what would he have to show to prove his Illinois residence?

I hope I've adequately asked my questions but I'll be happy to clarify anything further.

Federal Taxes: Temporary Living Expenses