I have had to change twice in the past year after one stopped working for me after 7 years. A person just has to hang on and make clear to their mental health professional that the medication they are on is not working and they need to change! Sometimes you have to be adamant.
Maybe just only ask him to do one thing right now, go to his Doctor, until after he sees his doctor. My first instinct is to huddle up and do nothing when my mental health is suffering, I sometimes simply can't do anything useful. One thing at a time. I would suggest telling him that things can get a lot better if he sees his Dr and gets a medication change. I can't tell you how much more my current medication is helping me than the last two, unfortunately it took some trial and error to get to this place, but that's part of the deal, unfortunately.
This is very hard, thoughts for you and your son. You are an angel for helping him work to get through depression, back in my day my folks just told me to snap out of it and that they didn't want to hear it. I am sure I have suffered from depression since at least age 12, when I vividly remember all joy suddenly disappearing with a dead feeling in its place. Your support will make a huge positive difference in your son's life, he's lucky to have you.