As John Lawler has taught us countless times (quoting from this answer):
Modal auxiliary verbs ("modals") are very irregular and have extremely
complex grammar. Every modal has two kinds of meaning: its Epistemic
sense and its Deontic sense.
Epistemic meanings are abstract and refer to logical predictions and
conclusions: This might/must/could/should/may/will/would be the place.
Deontic meanings are social and have to do with obligations,
permissions, and prohibitions: She may/can/should/must go to the ball.
must both are modals and both can have epistemic as well as deontic meanings. So yes,
will can mean
must doesn't always imply certainty (as in, "This must be where it took place", he speculated.)
According to me, your first example is epistemic. The speaker is predicting he's in New York, certainty isn't guaranteed.
if...will construction is that of the conditional mood, not the subjunctive mood as you've tagged it. It doesn't define limits of certainty.
- IF I give you 12 apples, you WILL have a dozen apples. (absolutely)
- IF you eat 12 apples, you WILL fall ill. (maybe, maybe not)
Bottom line, you should never rely on modal for certainty. Introduce other suitable words in the sentence.
If he is in the US now, he will possibly/probably/definitely be in New York.
EDIT: You can't use
will to speculate about the future, and you can't use
would to speculate about now. If you use
will, you'd be speculating about the present. See sense 6 in Oxford. If you use
would, you'd be speculating about the future.