My experience with collection agencies is that they are a complete waste of time. We have send 3 debts to a few of the "you don't pay unless we win" type places, never saw a penny, I'm not even sure they did anything to be honest. they get 20% if they are successful, which when you think about it, isn't enough for them to give a c#&p about.
If you really want to persue, I would write up a letter of demand, outlining what they owe you, how long it's overdue and whatever proof you have that they ordered the product from you (PO's, emails etc.) and what you will do if they do not bring their account into good standing in 10 days.
For even more impact, get your lawyer to write it on his letterhead.
Have you tried contacting him? he may be having a hard time financially and is too proud to admit it, maybe you can offer an installment plan, it may not give you the sense of retaliation you are looking for, but you may end up getting paid.