Is military school a good option for my 14 year old son who weighs over 320 pounds?

You don't provide any information about what individual and family counseling you have engaged in, which should be your first work to do, way before discussions of surgery or any other intervention, not to mention sending him to military school. You don't say how your son feels about this option, other than saying you think he would hate it and get teased a lot. It seems to me that the red flags are already waving in front of you to tell you that this is a bad idea. It is. 

Forget about military school. If you want to enroll him in a school that offers treatment for his obesity, take a look at Mindstream Academy. You can also look at Wellspring Camps.

If you haven't already done so, start with weekly individual and family therapy with a licensed counselor who has experience in adolescents, addictions, eating disorders and family counseling. 

If you are unwilling to set up long-term counseling for your son and engage in long-term family therapy while changing what the therapy reveals must be changed, drop the subject with your son and don't bring it up again.