Babysitter to look after my 11yr old son

Hi, I am looking for a babysitter to look after my 11-year-old son. I have 50% of the time on an ad hoc basis in the evenings when I have social or work commitments. I am not keen to leave him alone at night for more extended periods—3-4 hours. He is pretty self-sufficient, so just having a person to be here in case of emergency and for peace of mind for himself and myself. I am looking for someone open to ad hoc but possibly a regular night on those weeks.