Which sentence is grammatically correct is he with you or is he with someone else?

Here are the completed sentences for comparison:

1. Is he with you?
2. Is he with you, or is he with someone else?

Correct Sentence:
Both sentences can be grammatically correct, but they serve different purposes:

• “Is he with you?” is a complete, straightforward question asking if someone is with the person being addressed.
• “Is he with you, or is he with someone else?” is also grammatically correct. It’s a compound question that asks whether the person is with the speaker or with someone else. This sentence is more detailed and offers two alternatives.

The choice depends on the context, but both are valid and grammatically correct.