Can anyone help me with some pronoun confusion?

Whom is the object of a verb in a sentence.
Who is used for the subject.

Eg. Who asked you to the dance? (the 'who' person is doing the asking)

But: Whom did you ask to the dance? (the 'whom' person is being asked by someone else)

"Whom" is the object of the verb "should ask". Here's a trick: whenever you're confused about whether to use "who" or "whom" in a sentence, substitute the pronoun "he" or "him". If "he" sounds ungrammatical, then you want to use "whom"; if "him" sounds ungrammatical, then you want to use "who".

It's true that prepositions take pronouns in the objective case ("whom" being a pronoun in the objective case, and "who" being a pronoun in the subjective case) when the pronoun occurs in isolation (e.g., "With whom are you going to the big dance?"), but you need to look at the whole phrase that serves as the object of the preposition when determining the case of the pronoun occurring as part of that phrase.

The whole phrase "whom he should ask to the big dance" serves as the object of the preposition "about"; hence, you need to look at the verb in this phrase ("should ask") in order to determine the correct case of the pronoun in question. In "He was worried about who would accept his invitation to the big dance", you want the subjective pronoun "who", because it is the subject of the verb "would accept"; the phrase serving as the object of "about" is "who would accept his invitation to the big dance".

Look Dear,
I think this is just due to the preceeding "about".
You are always worried about whom?
The above example shows that instead of attaching whom to ask, try looking at its function in relation to about and the confusion is clear.
Hope i get your vote.

Hehe. Latinistic English grammar is funny.

Simple rule - "who" replaces "he", and "whom" replaces "him."

'Whom' is not the object of the preposition 'about.' It is the object of the verb 'ask.' The phrase in question is "he should ask whom." When in doubt about whether to use who or whom, try substituting they or them. Which sounds better, "he should ask them" or "he should ask they"? If "them" sounds better, use "whom", because both are objects. If "they" sounds better, use "who," because both are subjects.

Rephrase into a childish straightforward sentence:

He should ask who?
A should ask B?

If A is the "who", you use "who".
If B is the "who", you use "whom".

Now, make a better sentence, and memorise it:

Who should ask whom?

That's it!

"Whom" is used when the object of the sentence is unknown. For example, "I baked this cake for whom?"