how do you ask this question?

Hi,
I think I would use the first one, but I'm not sure the second is wrong. Maybe both are ok, but I like the first one.

Pchuanga. can you tell me when was the last time you saw him?

b. can you tell me when the last time you saw him was?

which one is correct?

I would say "Can you tell me when you last saw him."

I think YL's suggestion is good. And don't neglect this possibility:

Can you tell me the last time you saw him?

As for the word order of an indirect question with heavy clauses, you can be flexible. You don't need to strand the verb way at the end if it sounds very awkward. Your case b is borderline. It's the correct word order for the indirect question, bu

Of course "Can you tell me when you last saw him?" is much better, but since the question was a little different...
I considered the two sentences this way:

1) Can you tell me when was the last time you saw him? - Yesterday, Monday, etc. was the last time I saw him.
2) Can you tell me when the last time you saw him was? - The last time I saw him was yesterday, Monda

Equative structures are slippery.

Normally the subject is the entity to be identified, and the complement is the expression that identifies.

To my ear the last time you saw him is the entity to be identified, and an expression like Monday does the identifying. So to me the most natural order is:

The last time you saw him was Monday.

Yoong Liat

Pchuanga. can you tell me when was the last time you saw him?

b. can you tell me when the last time you saw him was?

which one is correct?

I would say "Can you tell me when you last saw him."

Yeah, I agree with Yoong Liat perfectly there. But if we have to make a choice of the two,

I understand that grammatically, b. is the correct answer.
at the same time, a. sounds more natural.