Since it sounds like you are on good terms with the owner, I would suggest you go have a heart-to-heart with him. Face-to-face. Not on the phone. Explain to him that you really appreciate/enjoy the work he sends your way, and you want to continue the relationship.
Explain to him that because he has so much outstanding debt with you that you really can't justify taking-on any more work on anything other than a cash-on-delivery basis. You could escalate it a little further by making it a "no delivery until the check clears" arrangement.
If he's a stand-up guy, he'll understand and want to work something out with you.
On the other hand, if he blows-up and rants about how he can get better work from someone cheaper, you'll know he's an asshole who's been stringing you along because he knows you're not a professional. Kiss him off and head to small-claims court to get the balance he owes.
Unfortunately, when you freelance, you run into a lot of guys who are out to screw you. It's just the way they do business.
Here's hoping he turns out to be one of the good guys.