Yes, "since" is the correct choice here: "I haven't seen my grandfather since the summer." The speaker saw his grandfather during the summer but hasn't seen him since. The period between the summer and now is a period during which the speaker has not seen his grandfather. We would NOT say, "I haven't seen my grandfather for the summer." However, if the summer ended three months ago, you could say, "I haven't seen my grandfather for three months."
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