First, there's no question that he hustles/escorts. You saw him walking shirtless in a hustler area and he asked you for money for what you've been doing.
So far, the money here is peanuts. Giving him $40 for 12 to 15 fucks, is like $3 per fuck. It sounds like he doesn't make a lot of money. Do you? Helping someone out with taxi/uber fair or something reasonable, from time to time, is not a big deal. Think of it like this... he fucks you and says, I'm starving, will you run down to the corner and buy me a couple donuts. You would have spent the same amount of money.
As for special things only you two do. I had a 2 year relationship with a porn star/escort. We did things that he never did in his videos. I have no way to verify if he told me the truth about what he does with his customers. And, you have to realize that you have no way of knowing the truth either. I think that is what's bothering you here... just what was truly special between you two. You didn't describe yourselves as dating or boyfriends so I'm guessing you're fuck buds. Therefore, does it really matter? If confronted, he'll tell you what he thinks you want to hear. Why? Partly, human nature. In a hustlers case, he doesn't want to lose a nice crash pad, good fuck and occasional money, gifts, food, etc.
You now know the truth. He's a hustler. Even if he lets you fuck him raw, there is no guarantee that if someone waves a few hundred, he won't spread his hole.
You said the sex is great. You have to decide if you're ok with a fuck bud who's being promiscuous, whether he's paid or not.
You have to decide if you're willing to give him cash from time to time. If you're not, you must be honest with him and let him know up front.
The 12 to 15 fucks could be because he likes having sex with you. Or, it's part of the 'boyfriend experience' to build a relationship and feelings so that you'll most likely won't say no when he needs momey. I believe the latter. Why? Because when you asked about being paid back, he could of said, I don't know, I'm really strapped for cash or I don't make very much money. Instead, he specifically said he wouldn't because you owed him for the prior fucks. He purposedly turned you into a paying customer and wanted you to know it. He might like you so you might get a big discount... but he clearly sees you as a meal ticket. And with enough fuck buds like you as regular paying customers, he won't have to walk shirtless for johns.
If any of this turns you off, don't hang out anymore. If you do hangout, ALWAYS watch your wallet and valuables. Keep little money in your wallet. Buy a lock box if you need to keep extra cash around. Why? Someday, he'll be in a pinch and he'll take all your money. That's what hustlers do.
I just want you to think about who he really is. Who you are to him. This isn't the movie pretty woman... you can't save him unless you can afford to pay his bills. And above all else, don’t fall in love! He's a hustler, fuck buddy. Not boyfriend material. That shipped sailed when he wanted money for fucking you.