Hopefully you have a care contract, and will do all the correct IRS filings as will he on his salary. May want to stipulate Room and Board is also included in this. Is the salary 24/7? Just be certain all things are completely and legally done, as the POA is responsible under the law for meticulous record keeping. I am assuming also that grandson is "capable" of giving this care.
Talk to an atty or tax acct to see what you need to file to make this legal.