Seconding the “Talk with him” advice. Ask him why he doesn’t want to take them. Most of the time, people don’t just start refusing to do something without cause.
Ask him to journal/write down the differences he notices days he takes his meds and days he doesn’t. Getting him to realize cause and effect for taking meds vs not taking them should help him decide if he wants / needs the medication. It might be that he feels the meds aren’t helping any more, as for long term medicating, the “new normal” can make you doubt that the meds are doing anything. It’s only going off them that makes it clear that they’re the reason for the new normal being a good one.
If he wants a med holiday to see how things are, set up a period of time for it and let him do it. Have him do self observation (and remind him to write down how he feels and what he got done that day in the evening, just so he doesn’t forget.)
Over all, you want him to feel that he’s an active participant in his medication plan. Allow him to have choice and ownership in the action. Medication is an emotionally complex issue, and it’s best to encourage him to decide his own course of action, and to talk out what he’s feeling. As a parent, I know you only want what’s best for him, but it’s often very difficult for anyone to see outside of their own experiences. Letting him test out medication vs non-medicated in a safe, supportive environment will help him out a lot in the long run, even if it’ll be very anxious for you in the short term.
Good luck with everything.