May I offer food or a treat to a Jewish soldier of IDF when I know he will not say a b’racha?
It is permitted to offer him food.
Sources: In principle, it is forbidden to offer food to somebody who won’t make a blessing over it (Shulchan Aruch, Orach Chaim 169:2). However, Shulchan Aruch writes that some permit offering food when giving food would be tzedakah. This would apply to a soldier, and even if it isn’t formal tzedakah, there is a mitzvah in giving him food, in bringing nearer Jews that are distant, and bringing unity to klal Yisrael.
Although Magen Avraham (6) writes that giving food for tzedakah is permitted only when the person in question might not make a blessing, it is possible that the soldier will make a blessing. Even if we are certain he won’t Machatzis Hashekel cites from Eliyah Rabba that it remains permitted to offer the food, provided that his not reciting the blessing derives not from wickedness, but from ignorance (see Mishnah Berurah 11). This is usually the case for secular Jews.
See also Shevet Halevi (8:17, permitting giving food when the negative reaction of not doing so might cause worse transgressions), and Teshuvos Vehanhagos (1:483, permitting inviting a secular Jew for the purpose of bringing him closer to Judaism, but limiting this to inviting him for a meal, and not to offering something extra.)