I'd encourage you to let this be a learning opportunity for your son. You don't say how old your son is, but I have learned not to try to get in and referee my son's friendships.
How I handle something like this is just by being empathetic, reflective listening and offering other choices. "Mom, Johnny brought candy and didn't share any with me." me: "Wow, how did you feel about that?" son: "Not very good". me: "Yeah, I don't think I would feel good either. Did Johnny say why?"
Son: "He asked me what I brought him!" Me: "Oh, really? How's about that?"
...
This sort of exchange allows my son to be the one to come to his own conclusions about how he feels via verbalizing his emotions. I also ask "So what could you do/say next time" and give a little gentle guidance... and then I let it go.
Kids need to have emotional space to come to their own conclusions/observations about their friends. If we tell them "Johnny is a user and a loser" and get mad *for* our kids, it's confusing for them, plus they get their feelings validated but without doing the work of verbalizing/thinking about the situation on their own, it's less effective. If you want your son to dump this kid who is being mean (or incredibly juvenile/manipulative), you have to *not get mad for him* and let your son get upset himself, realize he's getting the losing end--when he's ready to realize it-- and then he can decide what to do.
My guess is that calling the mom won't do wonders. If you are truly worried, do tell your son "stop loaning this kid money"... and then back it up on your end, mom. When your son does loan this kid the money, don't bail your son out or make it up for him. Let him deal with the loss. It's a life lesson. You put your money in an unsafe place/into unsafe hands and you are likely to lose it. And be glad that he is learning about users and losers at an earlier age and has only lost a few dollars-- your son will get a good little immunization to that sort of behavior once he realizes that this 'friend' is not a friend. But he won't get the lesson if you try to fix all of this for him. Let him know that it's okay for him to say no to this kid, to speak up for himself, and use other opportunities to point out what healthy friendships look like, so he has something to compare his to. Otherwise, try to stay out of it if possible. Your son will eventually get bored with this guy, or sick of it, and when he decides to do something about it, those actions will be authentically his.