That’s why my first advice is to let him know. He is about to be disconnected from their closest network, such as their family, friends, colleagues, etc., and be introduced to an entirely new environment. He must feel insecure, and he would much appreciate the assurance from you that you will, no matter what, be there for him and support him in any way you can after joining the army. This is why for Korean males, a final scene where they hug their family members, or their closest one in front of the military gate, right before the military enlistment, always remains one of the most memorable memories. It is a scene that spotlights the people that deeply care about them and be there for them, waiting for their proud military service to reach an end.
I would recommend not emphasizing how difficult the military service will be. The best you can do is be empathetic with him and try not to pass boundaries of making him sensitive. One last piece of advice is to make him feel excited. This would obviously be the most challenging part of making your brother feel better about it. Try weighting meanings to the military service that your brother is about to enter. A good starting point would be being introduced to new people who would be there to get through this and how healthy he would be by the end of his service.
I sincerely wish your family the best. A fresh new adventure awaits your brother, and an important milestone of turning points is ahead for your family..