My wife's mother has lived with us since October 2013. My son cared for her for every hour my wife worked. Can I pay my son and write off his pay as a tax deduction?
No... You could bring your MIL to a Elder Law Attorney and have him write up a care agreement in which your MIL pays both your wife and son. If you pay him from your own account that's not tax deductible..
If the need for care is substantiated and ordered by a physician, yes. You pay him an agreed wage, from MIL's funds, and you do the proper payroll tax withholding and issue a W-2 at the end of the year. (If you are in the US) You do not get the deduction, but the patient MIL does.