Perhaps you can tell him you're concerned about his health and that you are just as concerned that he takes his meds everyday? You can tell him that it's not because you think he is slipping but because you love him and don't ever want to see him suffer. His health is important to both of you and not taking meds regularly is an unhealthy habit.

As for me, it showed me that my boyfriend cared when he asked me if I had been taking my meds regularly. I loved that he asked but at the same time was shy and a little embarrassed.

Being asked everyday may make you seem nagging or untrusting, perhaps a gentle reminding question once a week?