Take a nice walk and sit down together and watch nature and let your mind, body and spirit, relax. Then just start talking/asking him heartfelt question.
If it was me, I would lay it all out and tell the person just how very much that they are to me, long term.
Just think about being able to sit across from him, in a decade or two having breakfast and knowing how each other thinks and acts without having to say a word, tell him that.
Always close one's eyes and see the future and tell him that that future includes him each and every day.
Now if he has mental health issues, be VERY supportive and tell him that anything and everything can be worked out TOGETHER, hand in hand, shoulder to shoulder.
You sound like such a wonderfully warm and VERY caring person and do not hold anything back, TELL HIM, just how much he means to you and that there is NOTHING that the both of you can NOT over come together.
Hugs, love and well wishes to you,
Walter