Accept this circumstance. In this situation you can't change it, so try to think only good thoughts. Think that time will pass and everything will return to normal, that's not the worst thing that can happen in life. Any situation must be perceived with dignity.
Maybe this separation will do you both some good, you will have a rest from each other and a lot rethink. Don't focus on what your young man goes into the army, and now life will lose meaning. It doesn't end there. This is only a stage that must pass.
On the wires don't cry and hang up on a loved one. Think about the fact that it is doubly difficult, as he was leaving into the unknown, parted not only with you but also with family and loved ones. You are at home, and it will be much easier to accept his departure.
After he leaves, don't sit at home, do not succumb to depression. Chill out, go somewhere to relax. Talk with your parents or friends. They will help you escape. Just don't stay alone with your thoughts. Immerse yourself in the work (study), leaving no time for sad thoughts.
Do not, of course, to embark on "breaking bad" to drown out the pain of parting. It would not benefit either him or you. Time will pass and he'll be back.