I’m very glad you wrote. We are by no means the last resort. You have a number of options available who have the potential for far more impact since they actually know you and your friend.
As you pointed out, at 16, there is little you can do on your own. However, you have a sympathetic dad and you go to a school where there is probably a counselor. Please, enlist the other adults. They can’t know how seriously your friend is troubled unless you tell them. Once they understand, they will probably be more effective advocates with his parents than you can be.
And, no. This is not a betrayal of confidences from your friend. When a friend has already attempted suicide and is asking for help, it’s important to respond — even if he might be mad at you for it. It’s easier to live with a friend’s anger than it is to have regrets that you didn’t help.
Your friend’s parents’ concerns are not unjustified. Psychotropic medications do have negative as well as positive effects. What they may not know is that there are a number of medications to choose from, some with fewer side-effects than others. Many doctors use medications only for a very short time while the person gets involved with therapy. Further, medications may not be necessary.
The parents have already shown that they support therapy. If your friend isn’t taking advantage of that, he should. Do encourage him to do so. Some studies have shown that when a patient really engages with therapy, it can be as helpful as medication. If the therapy isn’t working, the therapist is probably the best person to approach his parents.
I wish you and your friend well.