You have the right to set your own policy about maximum days he can be out sick, accrual of sick time, etc. If he is out too much, you can terminate his employment with you--he can still work for the staffing agency and they can place him elsewhere, but that is his and their concern, not yours. You are not required to pay for someone who is out or misses work more than you deem acceptable: you can tell the staffing agency that you are done with him, if he exceeds your policies and parameters. Remember: empoyment is "employment at will": you do not have to employ people who violate your policies.