therein lieth thyne problem - sorry, this computer is so old even the typing is olde style.
anyway, most window cleaners work monthly so if you were to go monthly (as opposed to 6 weekly) I think you'd have a much better chance of getting a decent reliable cleaner.
Also to give you an idea of price, we normally charge £1 to £1.50 per window, so lets say a double opener with a large window beneath will be at least £1 - so do a quick total of your windows and you will get a rough idea of what you should be paying (for exampe, a standard 3 bed semi with us is £8 to £10) If you were being charged less then thats another reason why he may not be cleaning your windows.
We always charge a minimum of £10 if we have no other customers on the same stop.
Hope the info helps.
Either way, drop him - tell him you don't want him back because underpriced or not he should stick to his word and keep cleaning your windows.