No sensible window cleaner would introduce you to all the customers until after you have paid the full amount.
I would suggest asking to help him for a day to see the quality of the work, and if it is ok then ask if you can pay half up front, work with him for a month so that you can do all the work with him.
Have an agreement written down and signed by both of you.
A letter signed by him introducing you to all the customers and explaining the transfer.
and then pay the other half.
Then you receive all the round details and any equipment included in the deal.
Also make sure that he will pass on any further customers to you, that might be rsponding to his telephine number.
I have done this in the past a number of times, when selling window cleaning rounds, and this has allayed any concerns the prospective buyer has had.