Because he is 26 years old, he will not have issues with the army. As long as he didn't live in Israel after age 14 for more than a year. While I don't know about revoking citizenship, I can tell you that Israelis who live abroad with dual citizenship have two options. They can get a passport, or a Teudat Maavar that only works in Ben Gurion. I think it is better to get an Israeli passport because I think it is the cheaper option. Also, it helps get you into countries that charge for visas for US citizens. a passport is for 10 years whereas a the Teudat Maavar is for five.
Whatever you do, DO NOT GET A TEUDAT ZEHUT. they will try to give you one. you don't need it.