Understanding "that" as in this statement

If I only say something as below without further more explanation, will a native speaker understand me?

He may be sexually dysfunctional, lately he can't do that his best. That's why I am here to help.

If I said it out loud and stressed "that", would you then ask me what is "that" ?

Solution 1:

Given the expression sexual dysfunction, any normal English speaker would understand it.

But I'm not sure what possible context could apply. If the speaker were a doctor or psychiatrist, he'd hardly be discussing such matters with a third person. If he weren't, it's not clear what why I am here to help would mean.

I originally answered before OP added the word sexual, which makes it a very different question anyway. All I can really say now is the sentence is not well-formed English in the first place. And I can't think of any better phrasing because the meaning being conveyed isn't something a normal English speaker would say in any context I can easily imagine.

Solution 2:

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."