Hi Andy
Don't contact his clients. It will create a bad business relationship between them and him and a possible one between you and he. Also you are not a dept collector.
I too am new to visualization, however I have run enough businesses to know that you should never go on trust alone. Always set up contracts clients and demand a deposite upfront. Your client, likewise, should have demanded a deposit up front before passing work on to others.
The idea of the deposit is basically for this reason. Whatever costs a service provider incurs are covered until the work is done. You are not a bank so don't treat your time as a loan. I've seen, too often, close relationships go south because of trust alone. I've also seen individuals go bankrupt because they placed "trust" in a client.
If he pays you himself then that is very good. If this doesn't work out then count your loses and use this as a lesson for the future.
When I started my first business, I trusted everyone and was "skrewed" by a very close friend. Since then I've learned, "because I trust everyone, I trust no one."
Hope this helps.
EDIT:
This is completely accurate. Some large firms get to a stage that they consider themselves untouchable. The will often completely ignore contract because they feel the other party does not have the means to sue. If sued, its a simple matter of throwing up their arms and saying "sorry, here's your money" or worse, counter suing.