What's your email address h20, I will send you a letter.
I bought a round recently and popped a letter through the door of all the customers that were out. The ones that were in i knocked and introduced myself.
You will need to know this guys name and a little bit about him for a letter to be affective.
Also for now don't change anything for the customer, keep the price and cleaning frequency the same, people don't like change and swapping window cleaners is already a big enough deal.
You can always change things and increase prices once you've gained there trust.