The money is overdue, and I sent a text message saying that I would go through legal channels.

Usually this is the last step, but you really don't want to pay back the money, so he can only go to court to sue. If you sue, you will definitely lose. Just pay back the money honestly. So in this case, you have to pay some money first anyway, and you don't want to pay it back at all. Offer to pay back the money, and he won't sue you.