Question

1. The boy is in the hospital.
2. The boy is in hospital.
3. The boy is playing in the hospital.
4. The boy is hospitalized for illness.
5. The boy came to the hospital to visit his grandfather. He is playing with his brother near his grandpa.
6. He is at the hospital.
7. He is at hospital.
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Does #1 mean #4 or #5?
Does #2 mean only #4?
Does #6 mean #5? Is it the same as #1?
Is #7 possible?

Answer 1

1 is correct in US English.
2 is correct in British English.
3 is correct.
4 would be correct if you put "an" in front of "illness."
5 is correct.
6 is correct.
I've never heard/read anyone using 7's phrasing.

I'd say that 1 and 2 both mean the same as 4.

Yes, 6 and 5 mean the same thing.

Answer 2

Thank you for your help.
When the boy is not sick and just came to visit his grandpa, can we say,'The boy is in the hospital?'

Answer 3

To make it clear, you might say, "The boy is visiting ... in the hospital."

You're very welcome!