Be empathetic but emphasize the positives. Maybe he’ll have an easier time fitting in where you will be? Maybe a better school situation or more time with a parent. Also I would try not to reinforce a sense of drama about it. If he senses you are anxious or upset, it will heighten his feelings- I know that is hard to balance with empathy.

We've only been in DC for 3 years and it's highly likely we are going to be moving overseas this summer.
Our 12 year old DC is so upset. He is an introvert, has a hard time adjusting and tends to be anxious about any changes in the routine.
Sigh...Any suggestions on how to make this transition easier? I was thinking about scheduling him with a counselor this summer for a few sessions before we move.