Possibly tell him what harm it is doing to you, how you feel about what he doing to himself. If he sees that people are there to help him then he should understand.
Also, you could do something that he likes, eg. Going to a theme park and then bringing the subject up when he is happy.
Finally, why not give him like a star chart. If he takes all his mess for a whole week running, then you take him to the cinema. But if he skips one time, he has to start all over again. Then slowly increase the amount of days and the size of the reward. eg. 10 days, cinema with friend. 1 month, theme park. Etc
Unfortunately, it's really hard to get somebody to recognize that they are not the same when they are not on their meds. My guess is he is embarassed of it. Embarassed of being diagnosed with this, embarassed because it confirms that he is different, and because he is still a child, he thinks if he doesn't take the meds, then nothing has changed and he is no different than everybody else. I can only imagaine how isolating that must feel. If at all possible, I would get him into some sort of counseling..........but do your research! There are a lot of quacks out there! Make sure you are picking somebody who's values are similar to your family and who is reputable. Not all counselors believe in the same things or work in the same manner so make sure you do your research. Anywho, if he had somebody to talk to in private, to build a new connection, he might be able to understand that there are many people who suffer from this and that in no way does it make him any different. I dont think he should have to be in therapy for years and years and years. But perhaps 2 or 3 months to get a grasp on the reality of it, and how to take control of it so he can participate in life in a healthier, less anxiety stricken way. Either that or you could try and have the talk with him yourself. But for some reason, kids dont believe what there parents say a lot of the time. I never did anyway......even though they were normally right. I was just sure that I was the exception to the rule. But if somebody from the outside came and told me, then I took it to be absolute. I'm sorry you have to go through this, and I'm sorry that he is struggling too. Mine are all still young, I cant imagine watching one of them hurt later in life even though I know that with 4......it's bound to happen. I wish I could be more help, but my thoughts are with you. Best of luck!