Is it correct the correspondence between probably/surely and should?

They are different.

The gunshot missed him. He should be okay except perhaps for shock.

"should" indicates that the circumstances suggest he's okay.

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probably he is ok

This is more tentative. It means we don't know. It could be that he is very capable and lucky so we ascribe a high probability to his being okay.

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he is ok, for sure

He is definitely okay. Of course this could be used as a reassurance to someone where there is doubt in the mind of the speaker but they wish not to upset the other party.

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EDIT 1 in light of new context.

I wouldn't use any of those. Here are my suggestions:

Somebody: How's John these days?

You: As far as I know he's okay. I haven't heard from him for a long time.

This indicates that you don't know and may not be very interested in John's fate.

or

Somebody: How's John these days?

You: I expect he's okay. I haven't heard from him for a long time.

This indicates that you think he is probably okay because "no news is good news"

https://dictionary.cambridge.org/dictionary/english/no-news-is-good-news

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EDIT 2 in response to comment by @Andrés Chandía

For me, the phrase, "he should be okay" is more likely to be used as a prediction, e.g.

Somebody: Look, John is about to go into that cave! Will he be safe? There might be a bear in there.

You: He should be okay. He is an expert cave explorer and will certainly be looking out for bears.