If I were you I wouldn't worry about it. You aren't deliberately targetting his customers, and if you are dropping leaflets, or even door knocking, you will inevitably meet people who already have a window cleaner. If his customers are happy with him, they will stick with him.
A funny thing happened to me last week. A young chap came up to me and said he was the local window cleaner, and said he did all the houses on the estate. I said I had picked up a few and was doing them, but was not looking to take any work away from him, but was merely targeting those who needed a window cleaner, but didn't have one.
He then said he would have a word with the other guy who does the windows (?) and said he would tell him to get lost so I could do the estate! The penny dropped that he was just checking me out. I told him not to do that as I repeated that I wasn't looking to take another man's work. He then shook my hand and off he went!
John