At 16 you can’t force him to swallow the pills, it has to be his decision. Its not that many years until he finishes school and will be out in the world either attending college or looking for a job and there will be nobody there to tell him to take his medication so he needs to make the decision for himself.
The first step is to find out why he doesn’t want to take them. As a teen I would also spit them out, and it was because I did not like the way they made me feel, in many ways they made me feel less like myself. I felt duller, like I had lost my spark. Maybe this is something that can be explained to the doctor the dosage may be too high, the meds might be the wrong ones.
The next key with teens is to find out what matters to them. As parents we want our kids to do well in school and not cause trouble but those things are not necessarily a priority to a teenager. But what goals does the teen have – be able to focus when playing sports, get into college, do better with friends because he can focus on conversations and activities, being able to enjoy hobbies like being able to sit through a movie or read a book etc. Perhaps have him try the meds some days and then not on others and see how they impact these activities that matter to him.
Meds are not the only treatment option for ADHD, would he consider trying some other treatments – biofeedback, elimination diets, cognitive behaviour therapy.
There are other things that can just be done in life as well which improves ADHD symptoms like reducing screen time, more physical activity, cutting down on junk foods, increasing protein etc.