a. He won't be back for six months.
b. He won't be back in six months.
Are both sentences grammatically correct?
Is there any difference in their meanings?
Does (a) imply that he will back shortly after six months have passed?
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Both are correct, and they have different meanings.
"He won't be back for six months" means that he won't be back during the next six months.
If he will be back at the end of the six months, you could say "he will be back in six months"; this does not contradict the earlier statement.
"He won't be back in six months" means that he won't be back until more than six months have elapsed.