As far as I know, Fattal is an American citizen only. He isn't an Israeli citizen. Even if the information came out that he is 'connected' to Israel, would the Iranians have treated him any differently? What would they gain by doing that?
Sen. Barbara Mikulski (D-MD) becomes 34th senator to endorse Iran nuclear deal (AP)
from the article: Israeli father of released U.S. hiker says his son is 'faring well'