I think it would be a big mistake to say you're looking. If there are some aspects of your current role that could be improved which might lead to you being happier (i.e. staying), then by all means discuss these with him. But don't let on about your search. Remember, you haven't secured a new role yet. If your manager knows you want to leave, he could give up on you, give you the worst roles, make it difficult for you to attend interviews, etc. So you could end up with no new job and your current one ruined. If and when you do secure an offer, then you can look to give as much notice as possible, but only do this once you're absolutely certain of leaving. Keep quiet for now.