Note that I am not licensed to practice in your state.
Lending money to friends is a horrible idea for exactly the reason illustrated by your question. You will lend money but you will ruin the relationship.
There are legals ways you can pursue with a local attorney to get your money quickly such as small claims court. However, in order to protect your credit score and so that the money doesn't go into collections, you need to find a way to make those monthly payments yourself. You can then go after him for the money. If you wait till he pays you to pay whatever company you owe and the payment is late, it will only hurt your credit score, not his.
In fact, even taking legal action is no guarantee that you will "get your money quickly". You may be able to get a judgment in small claims court pretty quickly, but does your ex have assets from which to satifsy the judgment? Does he have sufficient wages that a garnishment order would cover the monthly payments you need to make? If you enter into a payment agreement on a judgment, he might still miss payments, and although you would have some additional recourse, it wouldn't guarantee immediate payment. A garnishment order might be insufficient to cover your monthly payments, depending upon his income and the amount of your payments. Before you consider legal action, talk with a local attorney about the actual benefits you would gain the possible downsides. In the meantime, talk very honestly with your ex--if you "are still very close", try to make him understand the harm he's doing to you in terms of anxiety, credit rating, etc. when you can't make those payments on time and perhaps he will work a little harder to get you paid on time.
Have you asked him and what is his response? I assume you have attempted to do this reasonably or you would not have posted to this site.
1. Do you have signed promissory notes, checks or receipts?
2. Does he have money to pay you back?
3. You need to consider going to small claims court if your claim is within that court's jurisdiction and that way you may be able to avoid paying attorney's fees.
4. Last resort is to hire an attorney to see what the best way is to collect this debt.
Depending on the amount he owes you, you may be able to take him to small claims court. Make sure you provide evidence to the court of his the original obligation, his prior payments and what they were for, and what remains to be paid.