When should I tell him?

I live 4 hours away from a guy I met on holiday in the summer. We have been talking on fb ever since and the other weekend I had an interview for a work placement near where he works so we meet up and spent the weekend together. On the las day he said randomly so if u get the placement ul be here again and I said yes bet ur dreading it...he said no. So from that it's true he was thinking about the next time he'd see me. Thing is I've been told I've got the placement and its for a month starting in a months time. I dont know when to tell him. My friend said wait atleast 2 weeks but I dont know what do u think? He does work very long hours and some staff are off so I know he's even more busy than normal......should I wait or tell him when?... I want to see him bfre then but dont know how to tell him with out sounding desperate