Are these sentences correct:
1-He is the employee about whom I'd like to ask you questions.
2-He is the employee about whom I have to know what he does for a living.
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1 is fine if overformal.
2 may be grammatically correct, but it is so awkward that it is difficult to tell-- the reader doesn't want to bother sorting out a mixed bag of subjects and objects; s/he just wants it to be comprehensible:
'I want to know what this employee does for a living. No, not that one-- this one.'
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1-He is the employee about whom I'd like to ask you questions.
A more likely sentence is: He's the employee I want to ask you some questions about.
or: He's the one I want to ask you about.
or: I want to ask you some questions about him.
These are more likely in spoken language. "Whom" is not used often.
I think the context would say "he's the employee". The speaker might not really need to say that.