I think that if you try to talk to this other boy's mother, you're going to end up going down a very very long road with no end. In other words, your child is going to start hearing all kinds of things as he progresses through school. He'll hear rumors (one of my kids was told, by a young classmate, that you can "catch" diabetes from drinking milk), he'll hear inappropriate words and downright obscene words, he'll hear about sex (sometimes accurate, sometimes wildly inaccurate), he'll hear ridiculous jokes that are only funny if you're 8, and he'll hear remarks that can be hateful and cruel to a particular group of people (based on race, disability, appearance, etc).
Trying to hunt down the source, or alert other parents, will generally be useless. The exception would be if your child reports an actual threat to someone, an actual plan to harm someone, or someone who is being bullied who states that suicide may be their only way out, etc.
Instead, now is the time to start really strengthening your own child. Teach him how to ignore some things, not to repeat some things, how to stand up for someone who is being bullied, how to discern between something critical that should be reported to an adult like a school threat or when someone is talking about harming himself or herself, and how to talk to you about things he doesn't understand. Talk to him about modesty, about not discussing certain things like "peeing on someone" in school.
Don't spend your time wondering where your son's friend heard this term. Instead, spend your time helping your son know what to do with the things he will hear and see in the world. Make sure your son knows that if he hears someone talking about something that he doesn't understand, he can come to you, even if it's a curse word or a sexual term. You are available to listen, to guide, to inform. Help your son develop modesty, respect for all the boys and girls in his class, and the ability to be a positive role model for others, so that when classmates repeat what they heard from your son, it's all good.