First, to answer your question: No, I would not ask you what that means. The sentence makes it pretty clear to a native speaker what you're referring to, even if the rest of the sentence isn't entirely correct.
On a separate note, normally you wouldn't describe someone as being "sexually dysfunctional." That brings up certain connotations of disability that you might not be trying to express.
I'd instead say: "He suffers from a sexual dysfunction. Lately, he can't do that at his best. That's why I'm here."
But even then, it sounds awkward, because you're referring to the dysfunction in a euphemistic, vague way.
This is what I'd really say: "Lately, he has been suffering from a sexual dysfunction. That's why I'm here to help."